I think a fair move for the future of Smash would be maybe limiting it to a couple of franchises and billing it as a "PROPERTY A Vs. PROPERTY B,"
It would streamline things just by way of having two general "vibes," to develop off of. Let's say Mario vs. Zelda. You only have two artstyles to develop key art, environments, characters and music for. You can have your basic swordsman overdose with various Link types, your Mario sets. You have a couple of heavy set character types with Bowser and Ganon.
All the secret characters will be less surprising, but less of a pain in the ass to license. Also, limiting to two franchises would make most secret characters deep-cuts from said franchise.
Maybe towards the end of the game's life cycle, integrate a DLC character or two from the franchises of the next Smash up.
Fuck it, go all in with the crazy player feedback. Schedule twenty of these franchise fighters and look at the most played characters or highest voted from those twenty games and create a fan voted All-Star Smash roster for the cap off. First of it's kind. You could see a Zelda representative being Tingle with nary a Link in sight, should the data dictate it. Pay me, Nintendo. Pass that shit.
