I hate third party topics.........
I happen to hate Ridley topics but somehow they keep coming up...
Anyways, only third party character who has the best chance is Megaman. I mean seriously, he has so much history with Nintendo versus Sonic (Sega being Nintendo's former rival and all) and Snake it isn't even funny. The only major question regarding Megaman is which version would he be. While chances are it would be Classic Megaman a part of me hopes it would be Megaman X. Simply due to the fact that we have never seen the X series in any kind of crossover before. Another problem would be how do you represent the other series effectively when only one is in? My thoughts are a music track, somewhat similar to this:
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Classic Megaman Track
Megaman X Track (X Vs Zero in particular)
Megaman Battle Network Track (Nintendo could pull Starman's Theme from Network Transmission. Awesome soundtrack for a underraterd game.)
Megaman Legends (Flutter Vs The Gesselschaft)
Megaman Zero (Cannon Ball anyone?)
I would also like to see this done with Sega and Konami, except different games. Like for Konami, on Snakes stage give us one Castlevania track, Legend of Goemon track, something to that effect to better represent Konami and their other series. Similar with Sega; Monkeyball, Jet Set Radio, Samba De Amigo, etc... And if Square followed suit, we could see a Final Fantasy track, Dragon Quest track, Secret of Mana, Kingdom Hearts, Chrono Trigger, etc...
Problem with the idea though is that it might cause more licensing issues with individual companies sub-groups, so it might not be likely. Still, this is something I would like to see from third parties next time around.