Well if it is to be rebooted then I still stand in how I'd want the roster to look as I posted many messages above.
Still that's the approach I would be doing either way. Keep re-releasing, updating and enhancing graphics or an spiritual successor but that's just me.
The roster isn't going to matter in a game that plays differently. The whole point why people want certain characters to return is because they missed how they played. If the game plays differently than that idea is thrown out the window.
Mario will not be the same Mario. Donkey Kong will not be the same Donkey Kong. Basically if Smash is rebooted, the entire roster as we know it will be cut! This will be a whole 'nother beast. Who isn't in that game will not matter because Smash as a whole will not be the same Smash.
The roster doesn't mean crap in a brand new Smash starting from square one.
Also it's not a spiritual successor if it's an entirely different genre like an RPG. It would be a spin-off.
The biggest problem is the licensing issues. If Nintendo's unable to get some of the 3rd-party characters back, they can theoretically still expand the roster without them. The massive roster was a happy side-effect of getting everyone back as opposed to the goal itself, so there's not really much stopping them from making an even larger roster than Ultimate in the next one. Even if they have to make some cuts, they can always add newcomers, whether they be Nintendo properties or new 3rd-party deals to replace the old ones. I don't think there's any reason for them to downgrade when they can just do it again with some changes. I doubt we'll get everybody back again, but I have no reason to think the roster will take any significant hits in regards to size.
No they really can't. After getting a Smash that brought back everyone, no one will want another standard Smash that doesn't have their favorite character. You litterally can't top Ultimate. And adding more characters will continue to create balancing issues and put an increasingly greater burden on the developers. It's like you guys think they can continue to add an infinite amount of characters. As if that is feasible. It's not.
If you can't improve/top Ultimate then don't try. Just make something new.
You have to think about the future. Either things evolve, or they die. They can't continue down the same road forever. Especially if you a have culminated in the Ultimate Smash Bros.