AU where maybe instead of dying, Niki goes into years long coma and when he wakes up, the damage to his brain alters his personality? Like he becomes this super genuinely kind man who doesn't remember much. The doctors tell him about himself and Niki goes on an adventure to reconnect with his family. Imagine he tries to go to Scepter 4 because, "Oh, I have a son!" Fushimi thinks he's hallucinating again or something but nope they let him waltz right through the gates and the squad is like "it's your dad!" And imagine it's post-ROK so he was just finally feeling stable and comfortable with his life at S4 and with Misaki and this was the last thing he needed! I highly doubt he would want to even reconnect.

This explains that one short story AU with the flower shop and good citizen Niki, he has brain damage is that AU (that’s right, the only thing between us and happy Fushimi married to Yata running a flower shop is Niki getting dropped on his head). Imagine Kisa’s reaction to this whole thing, she suddenly has a loving husband and she doesn’t want him. Say instead of dying Niki’s gone into a coma and it’s likely he won’t wake up, Kisa has him moved to like a long term care facility that’s super exclusive but also way out away from the city so she doesn’t have to think about him. I could actually see that being stressful for Fushimi in this AU at least at first, always the worry that Niki will wake up and come looking for him (the jungle hallucinations would become even more traumatizing in this AU too, now that I think about it). Eventually he reaches a point where he just considers Niki dead though, he’s been told Niki won’t ever wake up and he sincerely hopes that’s the case.

So then many years later Niki defies medical science and does indeed wake up, but he has no memories and his personality is drastically affected due to the traumatic brain injury. Can you imagine the doctors telling Kisa that he may become violent or childish, he may end up someone who has no empathy, and Kisa’s like he didn’t need a traumatic brain injury for that. What happens though is Niki is now a nice, thoughtful, kind man, he doesn’t like seeing others hurt and he even wants to use his intelligence to help people. He’s all nervous and shy meeting his wife and Kisa can’t believe this is really her husband, imagine him taking a sincere interest in her career and she keeps waiting for the other shoe to drop. Niki gets to go home with like a care helper for a while and Kisa still largely avoids him but Niki’s so busy rediscovering his previous life that he barely notices, instead he just wants to reconnect with his whole family — which, of course, is when he learns about his precious son.

Kisa I assume knows Fushimi’s with S4 and even if she didn’t Niki does research until he finds out. He decides to go visit his son, all excited to meet the boy who looks just like him. By this point I assume someone has told him that pre-brain injury he was a piece of trash so he knows he’s a bit estranged from his son but still, he wants to meet Fushimi and apologize for his past self at least. Nobody at S4 knows that Fushimi’s dad is a touchy subject and since they look so alike when Niki shows up at headquarters it’s immediately clear that he isn’t lying about being Fushimi’s dad. Fushimi meanwhile is finishing up some work when Akiyama comes by to say his dad is asking for him and Fushimi just immediately freezes, going deathly pale. Akiyama notices that reaction and wonders if they should tell Niki to come back later when Niki himself walks in and greets Fushimi.

I just imagine Fushimi immediately pulling a knife and Akiyama has to stop him, like Fushimi-san wait this is your dad. Fushimi is breathing hard and his eyes are dilated, snapping at Akiyama to get out and take Niki with him. Niki says he understands Fushimi’s reaction — well, not entirely, he doesn’t remember anything, but he’s sure they can talk this out. I imagine Fushimi would just be entirely disbelieving of the whole ‘traumatic brain injury’ thing, even calling the hospital doesn’t convince him and he has to actually hack into their files before he feels even vaguely calm. Niki is being patient and caring about it too, like he leaves his contact info with Fushimi and says if Fushimi wants to talk he would love to talk. 

Even knowing about the brain injury I think this would be tough on Fushimi, this ghost of his past that he thought he overcame coming back. He feels like everything was just starting to get good again and he was almost starting to believe that things could stay good, and now in waltzes his worst nightmare — and his nightmare doesn’t remember him, and wants to reconnect. Fushimi is like vaguely aware that ‘this’ Niki isn’t really the guy who abused him but at the same time it is, he will never be able to look at Niki’s face and not see the guy who laughed at his pain. Ultimately I think Fushimi wouldn’t want to reconnect and would tell Niki to leave him alone, there’s just too much trauma there for Fushimi to reconcile even if ‘this’ Niki is a kinder person. I think he would maybe want to take this moment to finally get out what he’s always been holding in, I could see Yata or Munakata urging Fushimi to use this moment to say his feelings honestly to Niki just so they won’t continue to haunt him. Fushimi manages to tell Niki at least some of what the old Niki did and makes it clear he doesn’t need a dad now, he doesn’t need this back in his life, he’s trying to move on and he can’t do that if he’s still tied in any way to that man’s ghost.