All I'll say on the topic of cuts and returning characters is that whether the next Smash game (if one does happen) is a reboot or a continuation from Ultimate, this list of 1st party characters are who I firmly believe will 100% make the roster because of legacy, popularity, and relevancy:
These characters represent franchises that are definitely making the cut, but if major cuts happen, only one from each could make the cut and they have arguably equal chances:
and
If we do get confirmation of 3rd party characters, then it's a safe bet to see these guys return:
(Ken would only return if Ryu is base roster again rather than DLC)
Fox, Pit and Olimar are all debatable in terms of relevancy, but their franchises still have major points in terms of legacy and popularity. Marth is still very much the poster boy of the Fire Emblem franchise, despite his popularity likely being eclipsed by the Awakening cast. Pokémon definitely would have more characters, but it's too much of a debate over who would make the cut. The Smash 4 3rd parties + Ryu and Ken are all from companies who clearly have good relations with Nintendo, while other companies can have negotiations for their characters fall through, and the other Sega and Namco characters can definitely be swapped out, while these 4/5 remain, being pretty much pillars of gaming history and insanely popular.