As I noted before, Smash has gone in more directions than just Nintendo All-Stars(which as noted, even was shaky as soon as Melee. This is also partially when he noted the potential for a non-gaming character alone at that time that would be interesting). Smash 4 was the first one to hard designate it's for gaming characters only(and that was for DLC Ballot rules), and Brawl somewhat implied it with the 3rd party guidelines. Ultimate continues the Smash 4 philosphy.
It's not "always" been a thing at the end of the day, so it's a direction Smash absolutely could go in, or as noted, have different kinds of variable games that aren't straight sequels. Even if it's still the same core game and not an actual spin-off, they could have different versions for different purposes. One being about company battles(which means not everything is about gaming) is one direction. Continuing with mostly the same thing as Ultimate, just less characters is another. And a hard reboot is yet another. And there's countless other directions they could take it.
Ultimately(kek), the one thing that's clear is the idea of a port or returning everyone is not very likely at all with what he's said. He's made it pretty clear how difficult it is. Both, no matter how you go about it, is a huge development and money sink. And as pointed out by another user, they are going to focus more on bringing in new stuff this time around(as Ultimate is best to be thought of as a once in a lifetime situation). We also don't know what's going on with the Switch successor, and if it is backwards compatible, a port is a massive waste of time and money since you already pretty much have the full game. They would absolutely concentrate on a new game by that point. If it's not, then it has a slight chance of happening.
Though as the assets are just plain harder to work with than something like Mario Kart(and we know Pokemon clearly couldn't pull it off, heh), it's just a lot harder to keep trying to port it over. Code is old. It gets older as time goes on. Most likely at a point some stuff will require all new work(much like Mewtwo did in 4). Having Namco-Bandai there doesn't mean anything at the end of the day either. They're just extra help. They do not fix the major workload required even for a port. Reusing assets is not "easy". It's easier than new ones. It actually still requires a lot of work and time. It's not a "click this button and done" scenario. Development has never worked that way and never honestly will. Even asset reuse won't automatically work right away. A lot of coding is super easy to break, which means adding more stuff in can cause issues, regardless of whether it's "new" or not. There's just a lot to do.
Even when they ported over Brawl's assets to Smash 4, it's notable how a lot of stuff actually didn't work on the 3DS(and the Ice Climbers may not have even worked on the Wii U's 8-Player Smash. They even had issues with specials details like Wii Fit Trainer's fingers alone. That much stuff onscreen is unclear if it could've worked out). That said, it's not like they'd have been cut for being unusable in a single mode anyway, but being unusable in 3DS is different as both rosters were intentionally the same. It also cut Pokemon Trainer(with a returning vet of Charizard only) and forced other transformation characters to separate(which isn't a bad thing). Just having a strong system won't do enough either, as the big thing is how much development time is allotted. We know that having a lot isn't the end all, as shown by modes being ultimately DLC in Ultimate because of not having enough. If they were even base game to begin with(albeit, probably were?). If DLC wasn't planned in 2017, it's possible we never would've gotten said modes or it would've been pushed back to include that slight bit(unfortunately it would mean the last characters revealed altogether were Ken and Incineroar, but that's how it is).
will get cut?