Anything goes in this example for the sake of curiosity. Me personally I love obscure or not well received games. Because they are generally games that had an interesting idea. Just didn't have big budget or executed well.
So for crossover I'd like to see Rourke from Damnation into Brute Force. Had both series done well enough they could have gone far. Maybe Brute Force could have been the shooter equivalent of fighting games with guest characters from shooters.
Galen or almost any Ninety Nine Nights character into Soulcalibur as they would fit in perfectly well.
Krom from Bloodforge into Hunted The Demon's Forge as the two are both dark fantasy games. Also seem to be influenced by Gears of War with the style. Altho Bloodforge is more of it's own.
Master Chief from Halo going to Sera because maybe the Covenant traveled there on the great journey. And teams up with Marcus Fenix to take out the Covenant and Locust. Maybe the imulsion and flood team up or mix and create a terrifying threat needing the two worlds to work together.
I don't know if this counts as a crossover, but I would really love to see Lee Everett return in flashbacks for The Walking Dead: The Final Season.
