DCrooKZ

Personally, While Solid Snake is my all time favorite fictional/game/whatever charater, I feel he is slightly out of place in this game. I feel like Grey Fox, the cyborg ninja would have fit better with his crazy jumping ability and having a sword and all.

As far as 3rd party there are many great possibilities, Seems like they should have viewtiful joe in there. And Sonic.

Price of Peria would fit very well too, but Solid Snake vs. Sam Fishcer would be awesome. However I would let the entire staff of Joystiq punch me in the gut and film it if they put Sam Fisher in there.