I actually have though about this a LOT, and here is what I would pick...
- Resident Evil crossover with Street Fighter. An all fighting game.
- Resident Evil crossover with Supernatural. A game where you play as the Winchester bros. and Leon and Jill surviving an outbreak. That would be especially good because Sam and Dean are used to things that have UNnatural causes. Ghosts. Demons. The like. So a man-made monster would be all new to them, and possibly pose a threat.
- Resident Evil crossover with Marvel Zombies (Why not? Army of Darkness did it.) It would play like the Marvel Ultimate Alliance games.
- Finally, Resident Evil crossover with Evil Dead. Where you play as Ash surviving an outbreak, with his Chainsaw hand and Boomstick...