It's already telling how Smash had to reach out to third parties to generate true hype, but there's only so many of these characters that Nintendo can reasonably add without turning each subsequent game into a clusterfuck of copyright and removing content.
I'm sure there's a small number of reasonable characters to add still, not to mention new future characters. But everyone who had to be in Smash, is already in.
We may even reach a point where a reboot with brand new mechanics and a completely reworked roster is needed.
Well... that, or turning Smash into some Mugen or Naruto Ultimate Ninja mess. Which judging by rosterfags, might not be too frowned upon.