I'm pretty sure he'll end up in SSBR. Likely (hopefully) not Waluigi along with him. I mean, there's already enough Mario characters as it is... Dr. Mario should, in my opinion, be removed. He's cool and all, but he's not worth the extra effort on Nintendo's behalf. More Zelda characters are needed. You know how Bowser's the final boss of Adventure Mode? I don't see why. It should change depending on whoich character you're using--Ganon could be Link's & the rest of the Zelda Team's last challenge, King Dedede could be Kirby's (& hopefully Meta Knight's), a Metroid for Samus, Andross for Fox, Falco and whoever else will be on the Starfox team...
If Nintendo listened to each individual fan, there would be about 200 characters in SSBR. When a character idea pops in your head, think; how often would you play as Goomba??