3rd parties? Beyond the impossible option(Brian), I'd like to see a Microsoft representative(Master Chief, Steve?, Fulgore, or a Battletoad are my top picks, besides being the most feasible options), someone from Bandai-Namco(pet peeve is people thinking only Namco matters when both companies are combined, to say the least) like Agumon, Lloyd Irving, Nightmare, or Yoshimitsu, a Dragon Quest character like Slime, possibly Doomguy or Dragonborn, and I would love to see Scorpion or Sub-Zero, as they have interesting options for movesets. I'm not including vets when I say this, but Snake should come back imo. I'd like Simon too, but Bomberman is meh to me. Love the character, but my issue is trying to keep him interesting. It has a similar issue to what Pac-Man had. Pac-Man is actually pretty boring on his own and has few moves to work with as is. They had to make him basically an Arcade representative of Namco to make the moveset, well, work.
I won't much go into obvious issues with some of these characters, of course. Some are hard to imagine without their extreme violence, and I feel that is a big deal when it comes to representing them. It's a small issue I have with Ridley in 4(although I do like the fact he worked hard to make him act more like a playable character, with blatant Smash attacks. Not that I think the idea of you controlling him by... beating him up was a great idea, but still, it was an interesting idea in itself. OOC definitely, though). More specifically, making a character that yours can control, along with having an actual regular proper icon for defeating them, feels pretty neat and I feel he deserves some respect for the work he put into the concept.