While I personally think we may be done with 3rd Parties for the rest of Smash 4. I complied a bit of a list of who I think could be likely candidates based on our legendary or at the very least part of the mainstream consciousness that alot of people would be aware of. Using both already represented companies and those who could come in. Capcom is left off cause they already have Mega Man AND Ryu
Namco - Heihachi (There was something with Sakurai that said
he was considered at one point. Meaning it's quite possible he's next in line should Namco get the nod for another rep. He has at least a Mii Costume though)
Sega - Tails (Let's face it, Sega does have many good franchises but none of them are as universally well-known as Sonic is. A 2nd Sonic rep is the most likely possibility should they give Sega anything more. And I feel Tails would be the most likely choice being the Sonic franchise's Luigi, I think Tails definitely counts as an Icon of gaming at least in terms of sidekicks)
Rare - Banjo & Kazooie (Should be no question)
Ubisoft - Rayman (See Banjo)
Square - Lara Croft (Square currently owns her, and there are actually a few Tomb Raider games on Nintendo systems. Though she certainly doesn't have the cushion of Cloud having the Final Fantasy franchise to represent)
Konami - Solid Snake (As much as Bomberman or Simon Belmont would be nice. Snake has veteran status on his side)
I think all of these are possibilities should there be 1 or 2 more 3rd Parties in DLC now. I personally think Snake and Rayman would be the frontrunners here though. Already being in Smash one way or another (Playable in a previous game, and a trophy that still leaves people scratching their head in the current game respectively) makes me feel they'd be the most likely if there's more 3rd Parties coming at all. Again I'm not expecting any more 3rd Parties after Cloud, but that's so if it happens I'll be pleasantly surprised.