I actually think 'JoJo's Bizarre Adventure' is one of the best franchises for crossover stuff because the Stand system is so flexible. It's basically a universal translator for power systems. You can drop a Stand user into 'Persona 5' or have the Phantom Thieves encounter a Stand, and it just... works. The mechanics of spiritual energy and willpower make it compatible with a ton of settings. I've read a few where Jolyne ends up in the world of 'Devil May Cry', which is a fun clash of aesthetics, or where Diavolo's time-skip gets tangled with the rules of 'Fate/stay night'. The real fun isn't just the battle potential, though; it's seeing how the utterly bizarre JoJo personality and fashion sensibility warps whatever 'normal' world they land in.
My absolute favorite, hands down, is a massive fic called 'Wrought by Iron's Hand' on AO3—it crosses with the 'Iron Man' comics, of all things. Tony Stark builds a suit that can interact with Stands, and he gets pulled into the Part 5 conflict. The author nailed Tony's voice bouncing off Giorno's unnerving calm. It shouldn't work, but it does, because both are about flashy, tech-adjacent powers used by egotistical geniuses. More people should write crossovers with non-anime properties; the weirder the combo, the better it often is.