AU where Tobirama is found in a bloodline thieves base when he's an infant.

The Senju clan are the ones that raided them after they tried to snatch some of their members (the Senju may not have a prevalent Kekkei Genkai like the Uchiha or the Hyuuga, but they're all rather hardy people with good chakra stores, which is a desirable trait when you're looking to breed powerful ninja).

They find him after the fight and figure "well, he looks kinda like a Hatake?" so they grab him with hopes of strengthening their alliance with the Hatake clan by returning one of the kids they're notoriously protective of.

Only, he's not a Hatake. None of the Hatake recognize him at all, and no one has gone missing anytime in the last few years, not to mention he doesn't smell like a Hatake. So now the Senju are stuck with a random baby.

Butsuma just kinda shrugs and decides that they can always use more cannon fodder soldiers. He plans to give the baby to some other minor family to raise, but his wife calls dibs before he can.

That's the backstory to how he gets to his place as the second Senju son in this AU, though I imagine he's not technically part of the line of succession until after Kawarama and Itama die and Butsuma needs a second heir so that Hashirama will already have one when he takes over.

Now the main thing I wanted this AU for is that it lets us give Tobirama have any Kekkei Genkai we want (-Hatake, Senju, and any clan that has prominent physical bloodline traits ig?)

The obvious route would be "He's an Uchiha with Albinism", which is why they don't recognize him, but I know Tobirama-With-a-Sharingan is done pretty often, so I feel like doing something different.

So... How about Tobirama with the Ketsuryūgan?

I know in a lot of Tobirama fanfics he's able to manipulate blood, but I don't think that's possible in canon unless you have a Ketsuryūgan. I don't mind it because, well, it's Tobirama, if anyone could figure out how to manipulate blood even without a Kekkei Genkai, it's him.

OR! If we want to get even crazier (and way less canon, since it's only from a single anime filler arc) we could give him Ranmaru's Kekkei Genkai (it doesn't have an official name), which, from what I can tell is pretty much "make this throwaway character have cool abilities with undefined limits far past what's reasonable", and I think it'd be hilarious to see the Uchiha Clan's reaction to it.

If you want an idea of the unnamed Kekkei Genkai's abilities, it seems to be similar to the Byakugan in that it can see (and hear?) from miles away and through objects, and like the Sharingan it can predict movements, but it can also track chakra signatures, counter genjutsu being cast on the user, mask the user's chakra, make fake chakra signatures, use red dust to effectively negate other dojutsu and even read thoughts!

Yeah, see what I mean about insane abilities?

And guess what it looks like! ...It's just a red glowing eye. It's got no huge transformation, it just glows red. That's it. no swirlies, no bleeding, no nothing. Just... red glow.

Also Ranmaru is from Kiri, so Tobirama having a water nature makes even more sense!

Tbh I'm growing attached to the idea, just because of how insane the Uchiha would get about it like "wHAT DO YOU MEAN YOU CAN MAKE OUR SHARINGAN STOP WORKING?!"

I think Izuna would have a fit. He cannot believe his rival is doing this to him! How dare he! Stop cheating, Tobirama, it's not fair! he's supposed to be the one with the cool Kekkei Genkai, not you!! >:(

Did a lot of evil shit. But let's full face it these ain't one of them.

No one knows what anyone looks like. Madara has only been gone for about 60 years. People are still alive that actually met him and there are probably still artist renderings of him for history purposes. despite this, Nagato, Hanzo, and Onoki, who might not have fought him, but he was definitely a genin around the time Madara was still running around, all think Obito is him despite being a different height, different hair, different voice, and different powers. The only thing going for him is that he's strong and has a sharingan. and that's not even touching the fact that Naruto and Minato are the only two blonds in konoha not related to ino, and no one realized?

Medical ninjutsu. In the beginning of a series, it was well established that medical ninjutsu required both really fine chakra control and knowledge of the human body to make sure you aren't rupturing their lungs or giving them cancer. Naruto, who is established to have mid level control and 0 anatomical knowledge can just whip Gai up a new body. Then there's Obito who had literally half a body then got saved because Madara obviously has a medical degree, how else would he be able to integrate Hashirama's cells into him presumably before cell theory existed. And let's talk about eyeballs. Let's just assume for the sake of the argument Rin was a semi-trained medic who did Kakashi's eye swap in the middle of a warzone, and Kakashi still had a bum eye for about 25 years. Zetsu plopped Madara's eyes into baby Nagato despite a grown man's eyes being shoved into a baby's skull. Then Obito and Madara are playing hot potato with their eyes. Do Sharingan just have no nerve endings?

And speaking of jutsu that should be more important than they are, fuinjutsu. We know the Uzumaki had it and they made extra strong jinchuriki, that's why they were killed. We know you need it to make a proper jinchuriki or the tailed beast will pop out. and we know you need to write stuff on paper for it to work, that's why Jiraiya has his big scroll and Tenten has her twin dragon thingy. Except Minato can just touch stuff to make his teleport points. The sage can just plop the two most powerful seals ever on your hands and it's just two little temporary tattoos. Madara and obito don't need any seal because they can just plop the whole ten tails inside them despite not being uzumaki and there being no seals period. And you'd think for such a powerful and useful ability, more people would have it. it literally fits Sakura's mindset way more if she were a seal user way better than a medic.