Chimera-gui

Yes, factors. And even then, each material has their own isolated worldbuilding rules, which influence how the standard rules are portrayed or developed.

Starting with the basics, digimon has some kind of "natural growth" without the human influence, this is kinda depicted in Adventure (1999) with Tailmon -- she naturally evolved into Tailmon and is able to stay in that form after waiting for a long time to meet her destined partner, Hikari.

In 02 we also have the older six digimon staying in the adult/champion form mostly, because the time they spent in the DW (now in sync with the human world's time) has made them grow naturally into their respective adult forms.

"But Ni, if human influence is a factor, wouldn't they naturally evolve into another digimon without Taichi & co in this case?" -- Well, I suppose it could be a possibility if they hadn't human touch before? My thesis is that if there were other factors maybe it would've been something different, but i suppose there weren't any so yeah, they probably evolved into the forms influenced by their human partner factors.

But when you look closer, the first media digimon material featuring Taichi, V-Tamer, has something like this apparently: Taichi from that world evolved his Agumon (Zeromaru) into a V-dramon instead of any other digimon available in his V-pet. His influence in Zero's evolution route shows their bonds as a whole, because Zeromaru's evolutions seem to fit that Taichi's portrayal. Meanwhile the anime Taichi has Agumon evolved into Greymon and then going down into WarGreymon for another kind of portrayal. and the reboot Taichi goes kinda beyond that, as a mix of both Taichi's portrayals in a spiritual way, has this one not only have the OG WarGreymon line, but also the ability to evolve his Agumon into BlitzGreymon too.

How about Masaru then? He got an Agumon too! Oh, and Liberator's Ryutaro!? And this is where you notice even more evidence that the human partner is actually designed and developed as the digimon's theme and counterpart than something done in a vacuum. I dare to say, the human partner comes later and the digimon & its line comes first instead.

for the Taichi-trio examples, it makes sense that all three had Botamon → Koromon → Agumon in the first instance. But then, depending on the plot and portrayal, one Taichi got V-dramon while the other two Greymon. Idk if anime Taichi having Greymon line was a decision to keep them separated from the manga one (since the anime was definitely not a manga adaption for reasons), or anything, but i like to think it's because Yagami Taichi -- as a concept -- can convey many possibilities for a single digimon partner line.

But then, Masaru and Ryutaro and other human partners with an Agumon can give a different flavor and doesn't need exactly to follow the WarGreymon route. This is why Masaru's Agumon becomes GeoGreymon, and Ryutaro's becomes Tyrannomon instead. If you pay enough attention, both digimon lines seem to match with the human's design and portrayal quite a lot in those cases, and their evolution line tends to follow a theme based on both digimon and human personalities & bonds.

I've seen people in the past complain that Daisuke's V-mon should've evolved into V-dramon or something, but i think XV-mon fits Daisuke's design and personality than V-dramon's:

Daisuke is a friendly jock, he's rough around the ages and abrasive... but once you gain his trust and friendship, he becomes loyal to you and will defend you and make you move forward even if he has to say the cold truth in your face (had done that with Ken and Rui, remember?). XV-mon embraces his inner heart, the strong heroic-like imo. V-dramon doesn't fit imo, even if i like V-dramon as a whole! But V-dramon fits Rina's heart to me, based on what i know about her... and also V-Tamer Taichi's.

This is also another reason i don't agree with the idea Hudiemon would suit Ken better than Stingmon. I think giving Hudiemon to Ken is misreading his portrayal and shoving him into stereotypes. I think Hudiemon fits more Erika because of how it was developed in Hacker's Memory imo, and it's not because "girl tamer = girl-coded digimon" or "boy tamer = boy-coded digimon" assumptions. I don't want to spoil it, so... Please, play the game. Ken has a very scary side deep down which matches with Stingmon's assassin-esque portrayal, plus their epilogue scene is obviously a Kamen Rider Kuuga reference. Maybe now, in the 2020s and with more female Riders around, we could someday get a nod to one iconic fem Riders in Digimon franchise, right? Hudiemon has a Rider kick attack after all!! But part of me thinks Stingmon and Ken works better for that specific time and period, so it's pointless to argue about this imo... The epilogue obviously aged poorly and it had been getting a few "changes" in recent material like Kizuna and 02TB, but that's another subject for another time.

Even if this is not exactly confirmed, Daimon Chika's Piyomon has evolved into Aquilamon instead of Birdramon, and while at first i was a bit... confused, learning the digimon can evolve into ANYTHING with a few rules, i think it embraces Chika's inner self as a whole: a strong bird-like digimon willing to protect a small child, who then evolves into Garudamon still with that influence. I'd like to not believe this was a "dark evolution" thing...

Human potential can embrace more than one form too, see Amanokawa Hiro as example: Gammamon can evolve into three different specific forms based on how he's raised by Hiro. But without Hiro's influence... he automatically can evolve into GulusGammamon instead. All the other three forms have a "ring" on their wrist or paw, while Gulus lacks it. Said ring is theorized to represent Gammamon and Hiro's brotherly bonds, meanwhile Gulus is portrayed as this one solo survivor of his own species who's skeptical and locked into the "eat of be eaten" survival mode. With Hiro's influence, Gammamon is able to tame Gulus and properly grow, evolve into forms that embrace his friendship with Hiro, Ruli, Angoramon, Kiyoshiro, Jellymon, Espimon, Ryudamon, and etc in a healthy way and mutually.

Gammamon's example is basically the typical V-pet route: you get different forms according to the way you raise them, and then Gennai's quote on "there's no 'right' or 'wrong' evolution" starts to make sense.

Now, a personal note: I was the kind to assume "wrong" and "right" evolution stuff, to claim "this makes no sense" to a ton of routes, and to struggle really hard to finish lines for my favorite canon characters which lacked a full complete line like most of the main characters of Adv'99 and other series. I think it's a bit difficult to get this in the first instance, especially if you're used to Pokemon or other usual mon-colle franchises that doesn't have much freedom in design or more strict rules limiting stuff for the sake of consistency. But the moment i started to realize digimon can and will always evolve into anything, with a few general rules that can also be bent according to the story and worldbuilding, my ways to see this franchise changed drastically: Evolution lines that i thought were "wrong" or "nonsensical" started to not bother me, and i'd just try to research for anything they had in common, even if a little friggin pathetic trait.

And this is why i tend to be quite vocal and aggressive about not dropping a ton of nonsensical rules that never were stated by canon material or even from Bandai themselves. This includes how Armor and Jogress work, and digimon levels as well and other "design rules" that keeps making people see a new series digimon design and go like "sorry that's not a digimon at all" and then you look at the diversity in design and themes the whole franchise has saying the otherwise. "Looks fake", "Looks Pokemon-ish", i think it's ok to dislike designs and takes, but i don't think is right to mock them or invalidate them. Or would you like to have your fanmade creations denied by others like "this isn't Digimon enough!" and "that's more a Pokemon-ish thing!" etc? Sadly, this whole nonsensical rules has affected how I perceive my own fanmade creations and it bothers me how from time-to-time i want to make my own OCs and creations more "Digimon-like" when the truth is there's no such thing as "Digimon-like" at all when you get different styles and a diverse amount of designs & approaches from Bandai & their branches (like, the mons for New Century, for example).

ANYWAY, since this became a topic because of BeatBreak when people were eager on Gekkomon's evolution... Let's analyze the current main cast and their connections to their themes so far:

Tomoro & Gekkomon: Armalizamon's evolution theme was about Tomoro's desire to protect his family/Glowing Dawn and finally accepting Gekkomon as part of him. They might bicker, but you can sense they're way more closer to each other now. And about Monarchlizamon? It also embraces that desire to protect those, but also goes on a more independent route: While he's part of the GD family, he's also on his own ambitious quest to rescue Asuka. I think Tomoro being quite "noble" and "prideful" in this style meets Gekkomon's pure heart and desire to protect Tomoro and those he treasures a lot. Combine their desires and the very impsulvie Gekkomon with Tomoro's cold hearted (no pun intented i swear) and destructive nature... and you get this one form. But they're not using that raw power for malicious intents, so that's all fine imo! I'm not sure about Atratusmon and how this one will play, so i'll only keep this short.

Makoto & Chiropmon: Chiropmon's abilities fits Makoto's role as the technician and analyst of the group, since the mon can connect into Makoto's visor/glasses and send him information, like a spy drone or something like. Besides that, Chiropmon and Makoto's hearts are pure and child-like despite them being observant and smarter (respectively). NightChiropmon has more offensive power, but still follows the technician analyst themes, being able to scan stuff while still working as a fine secret agent-esque digimon (which fits Makoto's interest for sci-fi stuff imo), while ScourgeChiropmon suits the idea of becoming a full machine-like digimon able to make Makoto be more involved in the fights, especially when he wanted SO MUCH to be strong and able to do more. So, ScourgeChiropmon not only is based on the premise "wings to protect you" but also their promise on flying together in the skies someday.

Reina & Pristimon: This one is a whole interesting pair, because Reina is more a raw brawler in the sense of taking charge and being the "distraction" for the others, giving the offensive ones (Kyo & Tomoro) to prepare for a critical blow, or to give more time for Makoto to analyze the perimeter and scenario in order to set a plan and have their mission accomplished. Pristimon is portrayed as the "older sister-like" and is always caring for Reina, especially when Reina is down or feeling overwhelmed. Since Reina hates snakes, Pristimon evolves into Wolvermon, a mon based on the snakes' predators, Wolverines. Not only Wolvermon embraces Reina's abrasiveness and speed, she also works well as decoy and support unit. So... what does Bearcatmon gives to us? Reina and Pristimon's desire to be more active, strong and not to be left behind. Beatcatmon plays not only in the "small, but powerful" sort of theme, but also works under Reina's "instinct mode". The athletic demeanor and shape fits both Reina and Pristimon's speed and power. Also, i think Bearcatmon being more aggressive than the usual composed "vaccine-type" digimon is not wrong. There's vaccine type digimon that are quite aggressive af too, don't forget it.

Kyo & Murasamemon: Cougarmon embraces Kyo's desire to sleep more because Kyo hates to wake early, so Cougarmon is mostly sleeping. But besides that, Cougarmon shows a softer and kinder side of both Murasame and Kyo. Meanwhile Murasamemon holds a strong power and keeps limiting it, leaving said power be unleashed only when it is truly needed. It embraces both Kyo and his' cold nature, but also their noble air, the paternal figures of the GD family. But that's all i can tell from them, since we know so little about Kyo and Murasamemon's past. All we know is Kyo was not like the way he is now, and in the past he was way more cold and terrifying, seeking for strong enemies than his current goals now -- make a world where both digimon and humans can coexist. (i'll ignore the potential final/mega evo for Murasamemon, Habakirimon until we learn more about it anyways...)

And that's all i can tell about the subject. I don't care if people disagree with me, i just wanted to point out digimon evolution does have different factors in my opinion, and i'll try to write down more and those POVs i have based on my own experience with and studies in the franchise.