I just don't really see the point of a "reboot", even gameplay-wise, as long as Sakurai is in charge, which at least most of us presume he will be.
He's said in the past (unless I'm remembering wrong) that he doesn't like to change movesets drastically in order to avoid alienating fans from one game to the next, and doesn't like cutting characters if he can help it because every character has their fans, so if Sakurai's in charge, there's no reason in my mind to believe we'd get a massively shaved down roster and drastic character reworks (even if some characters could do with one imo like Sonic and Lucario).
In my opinion, the next Smash is gonna just be a typical new Smash game, bigger than Smash 4, but likely smaller than Ultimate by a fair bit.