What I find funny is the fact that no matter what, third-parties should be equal. I've looked around and the fanbase for another Metal Gear character or another Mega Man character is basically nothing, but whenever Sonic pops up, there's 600 character requests. Honestly, my opinion is that if there is a case that a third party gets another character, it shouldn't be from the same series. Smash is about diversity, and I'm sure there are plenty other SEGA, Capcom and Konami characters that can be introduced. Amigo and NiGHTS are all pretty well-known (not on Sonic's level, maybe on Team Chaotix's level if you think about it hard enough), but the other two companies do have other franchises to offer (Bomberman, Castlevania, Street Fighter, Phoenix Wright) but because SEGA isn't known for too much besides Sonic, which is EXTREMELY popular, everybody wants Sonic characters.
That's not saying I wouldn't enjoy them. I'd honestly be perfectly fine with Tails, Knuckles, Shadow, Eggman, Amy Rose or Metal Sonic, if I wasn't so picky about diversity. Hell, a fight between Luigi, Tails and Proto Man would be something to see. However, I think the "all-star" of the series should be the ONLY star from his/her series to enter the ring.