For those who don't know what a Crossover game is, it is basically where characters from different games interact with each other, like all the characters in Super smash Bros. This isn't JUST Nintendo games though, any games any characters.
I heard rumors of a possible Metroid-StarFox crossover game, which would be amazing. For me though, I think the best crossover game would be a Mass Effect-Halo crossover.
Heck, a Splinter Cell-Metal gear Solid crossover would be great too. Sam Fisher and Solid Snake snappin' necks? HECK YES!!!!!
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A good Crash and Spyro co-op adventure game.