Honestly, I’ve never really been big on crossovers. They work occasionally, but the whole question of “what would happen if X met Y???” always struck me as a pretty boring one. What would happen, without any particular plot to drive it along, is that the characters would meet, and that would be that.
Crossovers work best when there is a plot that involves both stories without seeming out of place for either. And the plot needs to be central to the crossover, or else it’s just character soup.