What would happen if tiftt!deku had ended up being rebound into a universe where he didn't get OFA? Like he ended up either not meeting AM or AM wasn't able to find him again after the sludge villain?
Okay, I've been thinking about this for a few days because this is a very interesting idea huhuhu
I think first and foremost, Izuku would feel morally obliged to get in touch with AM and warn him of the whole "if you pass this quirk on to a quirked person, you'll be killing them" issue. I think he probably wouldn't push to receive OFA himself, though, even though it would kill him not to have it — especially after all the extra bonding he did with the vestiges post- Coffin.
I just can't see Izuku, who is wholly selfless (and doubting himself like crazy after failing to save anyone Before), directly asking for OFA from All Might. I think he would bite his tongue and respect whatever decision AM made after the hero has all the knowledge he needs.
Following that conversation, I think Izuku would still get in touch with Nedzu — and probably Aizawa and Tsukauchi too, considering how much they are involved with everything — to forewarn them about the future. I believe Nedzu would still be on board with believing Izuku, purely because Izuku has proveable knowledge about things he shouldn't know (OFA's existence and caveats being a prime example).
After that, though, I think things would wildly diverge.
If AM hasn't passed on OFA, then I don't think he ever would. That decision would be wholly out of Izuku's hands. I really don't believe AM would ever consider giving it up if he knows passing on OFA will make his successor a target. If he's passed it on to someone with a quirk (or, really, anyone other than Izuku), I think he would demand to have it back.
If we assume Izuku acts extremely out of character and decides not to meddle with things after relaying the future, I could see the teachers arranging to try and still make it to Kamino so AM can face AFO with the intention to kill him. I think AM is suicidal enough that he would use every last ounce of strength to take AFO out, and kill himself in the process — but the biggest issue is I think this would ultimately be the worst outcome, because I doubt AFO would die.
AFO and AM are both significantly weaker than they were in their original fight, but AFO had six years to prepare for another face off. I definitely think he would be keeping a few aces up his sleeve. Garaki would probably have a way to revive him again. AM, though, would pretty much ensure that he kills himself in their fight.
I think this is where things could split again. OFA would be gone, so AFO could go one of two ways: either he takes the loss badly but has enough self restraint to restart his plan of societal domination, albeit WAY more aggressive now his main opponent is gone, OR; AFO goes off the deep end, throws a tantrum now that he will never get OFA / Yoichi, and rallies his troops to go full Scorched Earth on the world.
If we back all the way up, though, and instead recognise that Izuku, being Izuku, Will Not Let That Fly, I think things would get super messy, super fast.
Without OFA, and presumably without all the training Izuku did with AM prior to the Entrance Exam, there's no way Izuku can pass the Exam even with all his experience. An untrained body without a quirk to give him an edge would put him at an enormous disadvantage in the practical exam (Shinsou didn't pass, and they would essentially be on equal footing). If we assume he's still travelling back to the exam day, then without the vestiges to keep him on track I think his head would be too muddled to do well on the written portion, either.
After he tells the teachers everything and doesn't get into U.A., I think Izuku would either have to find a way to persuade his mother to do online schooling (which he would completely flunk, considering what comes next) or if that doesn't work, he would leave home. It would kill him to leave his mother, but he would think it's for the best. With the knowledge he has, he wouldn't consider it plausible to NOT do something to help shape the future.
Getting into the circles of 1-A would be hard, but with the knowledge Izuku has of his classmates, it's not impossible. I think he would try to get close to them so he can pass on knowledge of their capabilities from Before to boost their present day abilities way ahead of schedule. I also think he would probably go looking for some of the League himself, to see if he can talk them down ahead of them becoming a full group.
Jury's out on whether this would get him killed or not, though.
Obviously not being in U.A. creates a whole bunch of problems... either Mineta or Tsu (or both) would probably die in the U.S.J., or someone else depending on whatever variables Izuku's absense changes in that encounter. If the heroes don't take Stain out early, Iida would die. Bakugo's character development would be thrown completely out of whack because he would lose his rival. Blah blah blah...
I don't think Kamino would occur, ultimately, because I think too much would change.
...I don't think I can follow that particular plotline much further though, because I can see there being some overlap between what I actually have planned for TIFTT, and we don't want spoilers, now do we 🤨
Anyway, this was fun to ponder! Thanks for the question :DDDD
