Not sure if a reboot is the most likely outcome, but dang if it's not my most wanted thing to happen to this series...
Smash has gotten kind of stale. Not bad, but very predictable and, dare I say it... boring. I'm sick of all these unchanged veterans and all of these returning stages. Give me tangible, new content that pushes the series forward instead of maintaining the status quo.
I don't think it'll happen to such an extreme degree, but I am really, REALLY hoping for a hard reset. Throw everything out, start from absolute zero. Give us new movesets for the dozens of veterans in dire need of an overhaul and completely new stages that showcase a wealth of different themes, games, and ideas. Give us new mechanics; true assists, meter canceling, new specials, aerial smashes, literally ANYTHING, I do not care. Give us new modes and ways to play the game, as well as a good amount of single-player content. I do not care if the roster ends up being 25 characters, I'm that serious.
I am happy with Smash in its current state ending at Ultimate. The series could use a good overhaul.