Chrissyfer

-The timeline would be pretty much similar to the MCU timeline where Bucky gets turned into the Wintersoldier and Steve becomes Captain America, but the background of the alien part is based on the idea that the engineers indeed used our planet as a breeding ground for the Xenomorphs and that Hydra accidentally finds an inactive/collapsed breeding ground and is able to gather some of the DNA and preserves it while the Wintersoldier program runs in the background and Bucky clashes with Steve every now and then, but at some point, Bucky almost gets killed by Steve and Hydra retrieves his very beaten up body and decides that it's "time for improvements", by fusing his DNA with the DNA of the Xeno, hoping they would turn Bucky into the "deadliest predator ever", turning him into a killing machine that's mostly driven by instinct, which makes it too dangerous to use, so they keep it in cryo sleep most of the time.

-The messed up part about Bucky's last clash with Steve is that Bucky actually managed to kill Steve and that the USA had to choose a new person for the title of Captain America, a symbol to "inspire the country" and such (a propaganda figurine basically), not knowing that Hydra watches every step of the government, waking up the Xeno-soldier every few years to get rid of the latest captain America after letting him live for a few years (they're playing the government like a puppet basically).Then comes a period where no new captain America is announced and the Xeno-soldier remains in cryo sleep until the year comes where John becomes the new Captain America, because the country needs a symbol once again.This time, John isn't a supersoldier and the earth is very different because it's more advanced than the MCU version, since humanity is already capable of traveling through space and is about to set out on a voyage to a distant planet for colonisation, which is why they need the new Captain America to be "one of the first ones to arrive, as a symbol of hope for the planet".Of course, Hydra springs into action again and manages to bring Bucky on board, posing as a regular astronaut, until they pass the point of no return to earth and Bucky secretly begins to assassinate the crew in such brutal ways that it's impossible for the crew to think that this is the work of a human being and assumes, it's an alien instead that got on board somehow.

-Bucky's plan is to get rid off the entire crew first before he can take his time to slaughter the new captain America since it's his arch nemesis, but gets into situations where he's continuously being paired with Walker, who begins to develop a crush on the quiet man that's been pretty socially awkward because he spend most of his life frozen in ice but who, on the other hand, befriends him with his friendly and open attitude towards him.Bucky, who has been driven by the Xeno-instinct for most of his life, is now torn between his mission and natural drive to kill what he's been programmed for, and the parts of him that still remained human and who begin to fall for Walker, until he gets a intermission from Hydra that brings him back to his original mission, putting all thoughts about John aside.

-Eventually, the crews number is severely reduced and Hydra pushes Bucky towards finally killing Walker, who sneaks up on him while John takes a shower, who takes it as an "invitation" and Bucky's assassination attempt turns into a steamy make out session until Bucky can't keep up his human form any longer and turns into the Xeno-soldier.John can't believe his eyes and his mind puts 2+2 together and realises that Bucky has been killing everyone this whole time and also realises that he's definitely going to die now, but can't help but being aroused by the half human-half alien hybrid in front of him, who also can't shake the feeling that there might be more to John than just a potential target- a breeding mate.

-Both end up in bed and the Xeno-soldier turns John into his own little bitch, leaving Walker breathless and covered in wounds and bruises while it's literally the best sex he ever had. Eventually, Bucky reverts back to his human form afterwards and reveals who he really is - of course, Walker is shocked but since Bucky managed to spare him even though they shared the bed with him in his deadliest form, he kinda "shrugs it off"/hopes that there might be a way to avoid getting killed by Bucky in the future- since barely any of the crew is left alive and they were still on their way to a new planet, free from any humans, so there might be a chance of them living a "normal" life together.

-The journey goes on and John forces Bucky to make a promise to John to not kill anymore crew members because it could endanger the mission and fuck up their landing and Bucky agrees- under the condition that he gets to stay with John and “use him” every now and then, to which Walker gladly agrees. Some weeks pass by and John begins to notice some changes in his body, like mood swings, cravings and such, which earns him some jokes from the remaining crew that's just glad that the killing stopped mysteriously, about John “being pregnant” when they begin to notice that John's belly is growing and a quick check up in the medbay reveals that John is indeed carrying a embryo in a false uterus. The crew begins to freak out but the John keeps Bucky's secret to himself and so they're all seeing the pregnancy through together, with Bucky's Xenomorph side being overly excited while his human side has absolutely no idea how this could even happen because no one of Hydra would ever even have considered it.

-During a dramatic turn of events, when the ship has to pass through a meteor shower, John eventually gives birth to a surprisingly humane looking girl that barely has any resemblance to the Xenomorph, which disappoints Bucky a little, while John is relieved.

-As it turns out, the girl has more in common with the original Xenomorph alien then they would have thought, because she grows surprisingly fast and within less than a week, she reaches the size of a 4 year old at least, who is capable of producing facehugger eggs, the standard recreation for a true Xenomorph that wasn't artificially fused with human DNA. Everyone is clueless about the developments, so the crew went back to keeping the ship going, while John still needs some time to recover from giving birth, since his body has to break down the fake uterus first, when turmoil breaks out on the ship, as the facehuggers hatch, attaching themselves to some of the remaining crew members that Bucky immediately executes, to prevent the birth of another Xenomorph (for the sake of protecting John and also because he sees himself as the alpha, who couldn't stand the presence of another rival who would go after John).

-While Bucky goes on a hunt on the ship to get rid of the remaining facehuggers, trying his best to keep the remaining crew members from dying like flies, their daughter seeks refuge in John's room, like a terrified little girl being scared of a Thunderstorm in the night.

-Bucky then returns, covered in blood and gets back to John, to find their girl in his arms- a sweet sight at first but he has a feeling that something is wrong and is proven right when the girl suddenly tries to force a facehugger, that's been hiding underneath her clothes this whole time, onto the face of her mother, because a part of the girl can't differentiate between John- her birth mother and a regular human host, so Bucky is forced to kill her, after he and John fail to try to calm her down and to reason with her, breaking Bucky's heart in the end because he had to kill his own offspring.

-Everything calms down and the last few members of the crew who managed to survive have no idea that they're still traveling with a hybrid.

Possible endings would be;

A.) they make it to the new planet and either live together with the others while Bucky hides his secret

B.) they make it to the new planet and Bucky kills everyone before they land so he has Walker all to himself

-Starts in 2015 when massive interdimensional monsters emerge from the breach in the Pacific Ocean.

-Bucky and Steve join the program about 2 years after it was founded and become the perfect team, slaying dozens of Kaiju together and making a name for themselves

~3 years after they joined, Walker joins the program and has a few interactions with Bucky who tries his best to teach the new ones how to pilot a Jaegar but John, in his early 20s at that time, seriously dislikes Bucky and they both end up bickering and clashing every time they are forced to go on missions together, to the point where it even endangers the lives of other people (time duration ~4 years maybe?)

-In the meantime, John meets Lemar and becomes a tag team with him, climbing the ranks quickly

-In 2022/2023 a mission goes terribly wrong and Bucky ends up losing Steve, barely coping with the loss of his partner, he secretly developed feelings for and Bucky ends up taking a 5 month long break while John makes fun of him constantly for it, claiming that "he was never cut out to be a pilot."

-Bucky returns and ends up in one last mission, together with John who is now without a partner since Lemar had to step down thanks to the heavy side effects of the constant exposure to radioactivity because of him being a part of the program for much longer than John, piloting a Jaegar on his own before meeting John

-John and Bucky are a perfect match, drift wise, but not personality wise and their incapability to work together since John absolutely refuses to meet Bucky on the same level results in the mission failing miserably and John loses his left arm in the process, cursing Bucky forever and both of them being forced to retire, also because their failure resulted in the almost complete destruction of the Philippines.

-7 year time skip; John gets a special mechanical arm and secretly begins to experiment with Kaiju related drugs to deal with the pain and the recurring nightmares and trauma he got from the fight for the Philippines, not knowing that the Kaiju cells are slowly building up in his arm, making him part Kaiju, while being out of reach for the Precursors for now.

-While John still works for the military, but somewhere up in Russia (also as a precaution to keep him from interacting with Bucky) Bucky quits the military and travels the world, helping people who lost their homes during Kaiju attacks to get back on their feet, until Nick Fury, who is now in charge of the Jaegar program that was about to be shut down since the governments decided to focus on other plans that now failed, visits him and drags him back because the fate of humanity is on stake now.

-John and Bucky reuinite again, with both of them being "beyond thrilled" to see each other once more and with John never forgiving Bucky for the loss of his arm, which was more his own fault than Bucky's, while Bucky tries his best to stay civil with Walker, which is getting more and more impossible because of his abuse of the Kaiju drugs that have begun to influence his mind, since the cells have combined with his mechanical arm without anyone noticing (like in Uprising)

-The army tries their best to find them partners that are as compatible as they are for each other but fails in the end, so John and Bucky are forced to work together once more, trying to put aside their differences for the sake of the world, successful enough for both of them to develop some sort of "comradeship" which turns into a careful friendship over the following months, the drift bringing them closer together as they learn to understand each other.

-During that time, Bucky learns that John's "resentment" towards him is a result of him being jealous of him and also because his father used to drill John into believing that he would never gain his "approval" until he made himself a name like "Bucky Barnes", so John eventually began to hate Bucky because he knew that he would never be like him.

-John also understands that it's hard for Bucky to work together with him because he reminds Bucky of what it was like to work together with Steve- his sudden departure being something he never got truly over.

-While the drift slowly brings them together, it also raises more questions because Bucky notices that some of John's memories are becoming more and more "dark and blurred", as a result of John being more exposed to the Precursors while John begins to notice that Bucky's memories are showing more and more pictures of them together, laughing together.

-Before they can both address it, they're being sent to another mission to retrieve some Kaiju parts for the research of Tony Stark and Bruce Banner, the lead scientist of the newly founded Kaiju defence and John comes in contact with a fresh Kaiju brain that's still functioning which intercepts his brain through the cells in his arm, causing him to collapse with seizures when the Precursors try to force John into a drift with them.

-Days later, John wakes up in a makeshift hospital, hooked up to all kinds of machines to figure out how the infection with the Kaiju cells happened in the first place and with Bucky being severely worried about John, who just "shrugs it off", pretending to be okay, trying his best to hide the fact that he's responsible for the infection that is now visible on his left shoulder, neck, throat and parts of his face. Eventually, he breaks down and tells Bucky about the drugs, who is forcing him to throw them away, reminding him that he "doesn't need them to deal with the pain anymore" (double meaning) since he had him by his side now.

-John, still not quite right in the head after just waking up and with the cells infesting his brain, kisses Bucky in his confusion, backing away from him immediately but with Bucky being glad that it happened and they both end up sharing the bed together for a night (while also knowing that the next mission very likely being a suicide mission since Stark and Banner finally being able to figure out how to close the breach)

-Both of them getting ready to deal the beach "Gipsy Danger Style", by dropping a nuclear reactor and with John acting as the "activator" for the breach thanks to the cells in his body,- but the closer they get to the breach, the harder it becomes for John to keep his mind sane and to steel himself against the grasp of the Precursors who are more than happy about his "arrival".

-The Precursor voices inside his head are getting louder and louder and eventually, John snaps when they manage to fully take over, causing him to throw the entire plan overboard when they force him to attack Bucky, injuring him severely, before he gets back the control over his body and mind, realising what he has done and that the only way for this to end is together with him staying inside the breach when the reactor explodes and with Bucky trying his best to keep him from his suicidal intentions because he can't deal with losing yet another partner.

-In the end, John, who is visibly getting more and more corrupted by the Precursors, forcing Bucky into one of the escape pods, with Bucky being too weak to protest, so he's forced to watch how Walker stays behind while he gets send back up to the surface.

-The breach seemingly closes and everyone celebrates while Bucky is ready to drown himself in the ocean because he lost yet another love

-9 year time skip in which the world seems "safe" from the Precursors since the breach "closed", while Bucky is still haunted by the image of John who sacrificed his life for the sake of the world, until strange events begin to happen, that are supposedly caused by climate change.

-Neither Fury, Bucky or the two scientists believe that this is actually caused by the effects of climate change since the patterns are too similar to Kaiju markings, so they begin to investigate

-Bucky, who is sent to a village in Japan to investigate, suddenly sees a hooded figure that reminds him of John somehow, so he ends up following him up to Mt. Fuji where he makes the shocking discovery that John Walker never died and the rift never collapsed since the reactor never took off because the Precursors were able to get complete control over John's body, stopping the explosion in the very last moment

-Mortified, he's beginning to realise that the Precursors have been using John this whole time as their pawn so he could "prepare their way to dominate earth completely, while also covering the part of the terra forming for them to make the planet livable". Near the crater, Bucky is trying to reason with John, not knowing that the real John Walker has been trapped inside his own mind for over 9 years now, without a way out while being forced to watch how the enemy is abusing his body, studying his human anatomy to improve their own soldiers, creating grotesque Kaiju with the ability to pose as humans, secretly placing them all over earth while waiting for the right moment to strike, killing thousands of innocents in the process

-Their fateful meeting ends with John causing a massive explosion that ignites the volcano, setting up a chain reaction all across the globe and also starting a cat and mouse game of Bucky chasing after John to stop him from destroying the planet- and also to bring him back to humanity and into his arms again.

-In the end, and with a huge amount of incredible luck, they manage to stop the Precursors plans, making them furious enough to release their "last attempt of a ultimate weapon onto the planet", an incredibly grotesque fusion of 5 level 5 Kaiju, forcing John to fuse with them so his controlled body can act as it's "brain" since they still can't cross over completely because of the atmosphere.

-The mega Kaiju is being confronted by a team of marvel heroes (a bit like endgame/infinity war), their main goal being to free John from their control by ripping him out of the Kaiju's skull

-Brutal fight, many injured/death but Bucky manages to break him free in a dramatic moment when he is brought back, by the sound of Bucky's voice, trying his best to keep the Kaiju from moving so the Jaegar can strike- knowing that this would also end his life since he is more Kaiju than human at this point.

-As soon as the brain is ripped free again, another team manages to close the breach in the Pacific simultaneously and the remaining Kaiju begin to die off, collapsing one by one, the mega Kaiju also hitting the ground.

-The dust clears and Bucky wakes up again, after briefly passing out because of the fall and crawls towards John immediately, who is on the ground, breathing heavily and with his body still being fused with the dying Kaiju, until Bucky rips him out, finally holding him in his arms again.

Ending 1, John dies, also wants to die because of what he did