Devil May Cry and Bayonetta. It helps that both games were made by the same creator, and I don't see anything in Bayonetta (keep in mind I haven't played the second) that would deny any connection with DMC. It's just a matter of working with Capcom on making a crossover work.

I want to say Resident Evil and Silent Hill, too, but I'm not entirely sure. It would be awesome to see Nemesis versus Pyramid head, though. Maybe throw F.E.A.R. in there too so I can see my Alma and Alessa fight each other.