If Viewtiful Joe is in the game, he needs to just stay Viewtiful, with regular Joe as maybe an alternate costume or something. I'd love to see VJ, but I don't know HOW Slow, Mach and Zoom could be used without screwing everybody else up...
Unless maybe slow is a charge-up punch, like the Falcon Punch, Mach could be a rapid-fire barrage of attacks, and Zoom could just make Joe huge... Hmmm thoughts?
But yeah, Sakurai said to expect 2-3 more third party characters at the absolute most. If we got Sonic and Mega Man, that's all I'd need.