Ugh, I remember when I made a crossover story years ago (don’t try and look for it, that account is long gone, I was like 10), I think it was literally a whole fic where they just live in that universe. However, those are really hard to write, I somehow made 5 chapters before burning out. (In a random order as well lol, need I add that it was one fandom I knew well and another that I knew next to nothing about)
I’ve never tried something like that since, but it’s probably best not to do a whole fic if you don’t know what to do, maybe like oneshots or something silly.