If Kojima can convince big N to put Solid Snake into the game, who else should show up?
Tony the Tiger
Mickey Mouse
Kratos (fat chance, but it would be funny)
Sparkster
Gideon
Lewis Black
Blasto
Rockman
Sam Conway (Kagemushi)
Daalek
Jar Jar Binks
Hmm, who else...wouldn't mind seeing Mickey versus a Daalek in a smash match, myself... Or wait, Captain Blasto versus Lewis Black, even. The derogatory smart-assed commentary from both of them combined would know no heavenly bounds! (S'Too bad Phil Hartman's kinda dead, though)
Though seriously, like Snake,
Sparkster's an old Konami franchise too (and one I really, really dug, before Konami killed Sparkster so as not to piss off Sega/Sonic Team...grrr...).