I've thought about what it would be for a hypothetical Smash reboot, and this is the roster I made:
Obviously I can't project 5-10 years into the future, so I was thinking what they would do now. And some characters would be transformational (Samus/Zero Suit, Zelda/Sheik) others would have echoes (Marth/Lucina, Peach/Daisy) to make it bigger. Though, I still had trouble with sticking to 30, so I went to 40. Pokemon Trainer would have been Alolan Trainer (or whatever future Gen it is). And I feel like this is way more third parties than would be in a hypothetical roster, but I wanted to give each third party one character. Also I could see one or two of the retro characters being replaced by a WTF character, but can't think of any good ones (which is why I'm not a game designer).