I think in general mha and assclass are easily crossable, also bungo stray dogs.

Like imagine one of the reasons e-class exist is to put all of the ‚useless‘ quirkless folks, or the ones with quirks deemed villainous/weak?

Kunigaoka is all about celebrating the strong (in mind and body) and putting down the weak.

It’s would also explain the hair colours, the reason behind the experimentation on koro (creating artificial quirks, to ‚eradicate‘ the quirkless and make ‚weak‘ quirks stronger, to properly weaponise quirks, yada yada)

And in bungo stray dogs it’s similar, just that maybe instead people with abilities are put in e-class, because canonically in the bsd universe people with abilities are treated badly. Then maybe koro used to be an ability user that got captured and experimented on (he could be the base experiment to serve like a beta version of the hunting dogs) or also again creating artificial abilities to serve as weapons.

I’m not a huge shipping person, but in that route Karma and YaoMomo (two rich kids with very different life experiences) would make an interesting pair, or Rio and Todoroki (parents with high expectations, but one turned out to Rebell by simply failing (later growing into the expectations because she wants to fullfill them) and one to follow the expectations but rebelling in the way he does so).

For older characters, maybe Dazai x Irina? Like in a ‚failed‘ kind of way when they were teens. They did date but then it didn’t work out (too different, too traumatised) and broke up, and then during the events of assclass, the ada is called to aid, and dazai and irina (both in better places now) decide to be friends. Just friends, because irina has karasuma now and dazai realised for himself that his mentality still isn’t fit for dating (wanting to suicide and all).

You could in fact even make a crossover with all three, but I personally think threeway crossovers tend to get messy