Okay, now THIS is an interesting question:
If you were Sakurai, what third party character(s) would you try and go through to get in Smash?
This is either really easy or really difficult for me, since mainly the only remaining characters I'd like are third parties. And in general, I think the ones I want make the most sense for Smash, but I can't completely remove my biases.
Banjo & Kazooie (Microsoft) and
Crash (Activision) are the big ones. The former is a former Nintendo character, so their lineage is stronger than most third parties, and the latter because Crash is the most iconic platformer not yet in Smash. Both fit in well, move wise, and very popular among the polls.
Outside of that, I think
Sora (Disney/Square Enix) is the biggest third party and would create waves with Disney involved. Extra points if I, as Sakurai, could somehow sneak in Disney icons, but not necessary.
Rayman (Ubisoft) is another very popular platformer, who's popularity rose during Smash 4 thanks to the fake leak, and Ubisoft would probably be one of the easiest companies to work with. Finally, while my personal Capcom rep is Dante, I feel like
Phoenix Wright (Capcom) would be a fan favorite given his position in the polls, as well as his strong history with Nintendo.