I think Smash Ultimate really will be the last Smash game in its current form. If they want to keep doing Smash it needs to be completely fresh. It needs a new genre entirely. They should add the third dimension and switch to an isometric view. While this would appear to be a RADICAL change at first, remember that Kirby made the transition in Forgotten Land just fine. However, this would also result in a massive roster refresh, not only justifying how many character wouldn't make it back but redoing the kits of existing characters to fit the new style. It would be exactly what the IP needs to reinvent itself without losing too much of its original identity.