Man, I've spent way too much of my life thinking about this. The most obvious crossover that just works is 'DanMachi' and 'Fate/stay night'. The idea of Bell getting caught up in a Holy Grail War, maybe with Ais as his Saber, writes itself. But I'm more interested in the weirder ones people don't talk about. Ever read a good crossover with 'The Rising of the Shield Hero'? Bell's pure-hearted optimism crashing into Naofumi's bitter pragmatism could be brilliant. You'd see Bell desperately trying to prove not everyone is terrible, while Naofumi reluctantly starts to believe him. It’s not just about power levels; it's that clash of worldviews.
A real deep cut I saw once and loved was a 'Kino's Journey' crossover. Kino arrives in Orario, not as an adventurer, but as a traveler just observing this bizarre, vertical city with its own rules and gods. It would be a slow, philosophical slice-of-life piece from the outside looking in, full of quiet moments where Kino and Hermes just talk about the nature of the dungeon. Nobody would read it, but I'd adore it. Honestly, I'm tired of the usual Shonen mash-ups; give me the quiet, character-driven crossovers any day.
I have a soft spot for stories where the crossover character completely disrupts the power structure without even trying. Imagine someone from 'Mob Psycho 100' like Shigeo just showing up, his overwhelming psychic power reading as some insane mystery skill, and he's just... deeply uncomfortable with all the attention and wants to work a normal job at the Hostess of Fertility.