They are going to need to make roster cuts, especially for the third-party characters. The game will probably have one or two more characters overall than Ultimate, but a lot of cuts. Or it could have five less fighters overall, with some newcomers and a lot of cuts.
I don't think we're going to go from 82 fighters (89 including echoes) down to under 60 like many suspect.
Online needs to be improved, but this is dependent on future hardware improvements for online. If Nintendo continues to have trash online, Smash will as well.
There's a very good chance the next Smash isn't really a new Smash at all, and just Ultimate DX with a few changes, a few new fighters, a few new stages, and better performance. Mario Kart did it, why not Smash? Though the big difference there is that Mario Kart 8 was held back by the failure of the Wii U. How much sales potential could you truly get out of Ultimate DX? 10 million units? Yes. 20 million or more? Highly doubtful. Plus, Mario Kart 8 (with or without DLC) still had some opportunity for more content. With Ultimate, it's such a jam-packed game already that slightly more content isn't much of a hook.
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima