I think all of the recent DLC talk is rather telling that we're starting to hit the bottom of the barrel for "obvious" Smash character choices. By obvious, I mean in recognizable names that most people would think such a character would have a chance or would have already been in the game by now. That's pretty much just Dixie Kong left as far as newcomers go and Chorus Kids by virtue of Gematsu. Ridley was in that previously 4-man band but has since taken to stage boss status (I still think they can use one of Ridley's many forms for a player character, including the most popular of them being his Super Metroid look).
At least, until new IPs come around to showsteal and present their characters as new obvious choices.
I highly doubt we'll get any more 3rd parties, even as a stretch for the next Smash Bros. I've even wondered myself if and when Sakurai ever decides to stop making Smash games from sheer burnout or whatnot, and that meaning no more Smash Bros or someone else takes over the job. Sakurai IS Smash Bros, even if we all know he's not soley responsible for the series.
As for Smash 4, I am not sure if we are getting characters after Mewtwo, but I could be (and hopefully am) wrong on that. Wolf, Lucas, Dixie, and Chorus Kids are the most likely, with Inkling being a highly requested yet unlikely one. We might just get stages/music but who knows at this point.