I thought about this a bit but this might be the worst Idea so far, but considering the "book" 's title you'd already expected it so here you go.
Basically I've been seeing a lot of people getting inserted in oshi no ko (probably because everyone loves Ai and doesn't want her to die) so here is my pitch. What about Miguel O'Hara aka Spider-man 2099? He originally tried to replace himself from an another universe for a family, but we all know what happend. What about an anomaly appearing in that universe, miguel having to rescue Ai, almost dying in the process and getting saved by Ai in the process (last one I doubt, because Ai probably doesn't have medical skills and he can just call for backup with LYLA, and he also has regeneration)
The problem is about what happens after that, because I'm not a good writer. I just want Miguel to be happy I guess.
I love happy endings. (Doesn't mean I hate bad endings but anything happy makes me happy)
Off topic
Steve Harvell, the guy that sung "All Star" has only a few days to live because of liver failure. Honestly I am really sad about this news, considering it was kind of a song that lifted my spirits up when I was sad. I don't know why I feel the need to write this honestly.