A crossover that would be interesting, and even has some precedent in games, would be a Luigi's Mansion and Castlevania crossover. You could have King Boo and Dracula teaming up with one of them, likely Dracula, taking control of the show sometime in the middle of the game, with Luigi having to face him. He could have multiple Poltergusts from previous games, with the 3000 version (the one in the first game) being used to deal with the physical undead by burning them, freezing them, or dousing them with holy water, while the others are better for dealing with ghosts and other spirits.