Hey, a question just occurred to me!
What would have happened if instead of Kou, Tajima had died? More specifically, how would Kou have handled the whole Madara befriending a Senju thing? How would the river confrontation have gone?
It has nothing to do with OOT but I find Kou to be a very interesting character and I would like to see how different the canon would be if she were alive instead of Tajima
I'm so touched you find Kou an interesting character, I really can't overstate that enough. The highest compliment for what's fundamentally an OC 🥺💖
Tbh, my first thought for the 'what if Tajima died instead' was could there be a river confrontation, would Madara and Hashirama have met and befriended each other in the first place? Mostly because if Tajima has no siblings the next person to be clan head would be Madara...but having a literal child be clan head would be a monumentally bad idea, no matter how powerful...damn now that'd be a really cool AU idea...anyway. Assuming that Kou became interim clan head or someone else was until Madara was older, I think she'd have really conflicted thoughts and feelings about Madara and Hashirama.
My characterization of Kou is usually she came from the 'weakest' Uchiha line. The kind of Uchiha that never awakened sharingans and had little talent for shinobi skills comparatively. This is a Major Lore Thing in OoT, but is a general headcanon for the character I'm fond of. Her insane talent and strength seemed to, quite literally, come out of nowhere and it made the more established branches of the Uchiha uncomfortable with her. Tajima never cared, my favorite headcanon is that he was a secret romantic at heart, and developed a crush on Kou without knowing exactly who she was. He figures it out pretty quick, but that willingness to not focus on Kou's situation, either coveting her power or being envious of it, was what made their relationship work. I'm very flexible on exactly how they get together, I don't think I've covered it in detail in any AU, but sometimes I like to think Tajima rebelled against the elders and chose to marry Kou or that they had an arranged marriage that both actually wanted, depending. Either way, I always like to imagine Kou being on the outs with the Uchiha elders/tradition and their wariness extends to her children and this is, part of the reason, Madara eventually had his own poor reputation amongst the clan. He was constantly in danger of being seen as Kou's son, not Tajima's and being a sort of usurper like her. A 'damned if he does, damned if he doesn't' situation that, unlike Izuna, he couldn't smooth over with charm.
This is a lot of background to say I think Kou would be more open to the idea of Madara being friends with a Senju than Tajima or other Uchiha. She's fought and killed them, and has lost a lot of shinobi friends, but her immediate family wouldn't have suffered losses directly from the war with the Senju. However, she'd be (rightfully) terrified that if the clan found out, they'd think Madara a traitor and Kou wouldn't have the political power Tajima would have to protect him. She wouldn't threaten Madara like Butsuma did Hashirama, but she would confront him and try to put a stop to it, though again, her worry would be for Madara's safety more than anything else.
I think I'm more fond of Kou as a genjutsu specialist than a katon one, so, if we were going for a darker AU, I think it's possible she would either try to put Madara or Hashirama under genjutsu to break them up. Genjutsu is just an illusion, one application would be to make Madara and/or Hashirama believe they've had a fight and the other calls off their friendship. This would be Kou's preferred method, much as she'd dislike the idea of lying and potentially using her powers on Madara. Tbh, she'd probably target Hashirama instead for those reasons. However if that didn't work/the river confrontation happened anyway...tbh fam I don't think she'd bring Izuna and if it was just her facing off against Butsuma and Tobirama face to face...she'd kill them, or at least definitely Butsuma who probably killed Tajima in this sort of AU. He and Tajima were of equal strength but if Kou got him in a genjutsu, it's over for him. To me, she's always the 'strongest' Uchiha before Madara, all of her issues come from political ones in the clan, mirroring what will eventually befall Madara bc I love me some symbolic 'like mother, like son' machinations.
If Butsuma's dead that could lead to a power struggle between any possible siblings/Hashirama/a regent ruler, which could be an interesting 'accidental kingmaker' sort of AU. If we're going for the more crackish approach, I think Kou would 100% be willing to kidnap Hashirama, put a shitty black wig on his head, and say she found him in the woods and is keeping him because Madara's fond of him. She'd say it in a way that implies Hashirama is a stray bird, and everyone in the clan would watch the wig slip from his head but be too stunned to call her out on it. A different more serious scenario I've thought about is the river confrontation accidentally becomes a Big Deal, as in the Uchiha clan finds out about Madara meeting Hashirama and even if she kills Butsuma, Madara'll still be considered a traitor, so Kou absconds with him and Hashirama both and they grow up basically clanless from there.
I dunno there's a lot to do, and this feels kinda rambly, but I hope it satisfies your question!
...I sort of want to write some of this properly now. RIP