Rather than speculate, I have a proposal. Other than the primaries (Mario, Link, Samus, etc), make the rest of the roster brand new fighters.
It's been 19 years since Krystal came into our lives. Sakurai of Smash Bros fame talked about this on Twitter.
Masahiro Sakurai said that the final Super Smash Bros. 4 roster was fully decided two years ago because of development time issues.