I honestly don't think any Smash from now on will have a roster smaller than 60-70 characters. So if that's around the maximum size, I'd like to hear your thoughts
Okay, so the parameters here are a roster of about ~70 characters, a total reboot with no grandfather priority, and me calling the shots.
Now, I'm not doing a full roster here but just four franchises (and I also don't have the in-depth details on budget/resource constraints like the actual dev team does), so if I were to fully complete this up to 70 characters then some things could change (maybe one of the last couple picks gets bumped or something), but here's more or less what I think it should look like with this roster size. I'll try to list more or less in order of priority, and remember this is more personal preference than what would probably actually happen.
Lastly, I'm not going to include characters from unreleased games, so "Scarlet/Violet rep" and "next FE protag" won't appear here. If you want to imagine them, then they could be either simply added to what I have here or in place of one of the lowest-priority reps.
MARIO
Mario
Bowser
Luigi
Peach
Rosalina
Bowser Jr.
Waluigi
Daisy (Echo)
Toad could just as easily fit in the newcomer slot as an alternative option. Not much boat-shaking here, actually.
ZELDA
Link
Zelda
Ganondorf
Impa
Skull Kid
Mipha
I tried not to cheap out by just putting "a Champion" at the end, but it's kind of a tossup between Mipha and Urbosa. But, on the whole, I think this is just way better than 4's or Ult's Zelda rosters with like the same number of characters. Of course, it's more work with the additional uniques instead of the Links, but I think it could be fit with a roster significantly bigger than base 4. If there is extra room for a second one-off like Midna or Ghirahim, then even better (they would be next up, but I'll be more conservative with the rep count here since this is not a full roster).
The single Link can either be just classic or have the alts split between classic and BotW, while Zelda would be designed more or less as she is in Ult, which I think well-represents her typical look throughout the series. The amount of changes to Ganon/Ganondorf depends a lot on what the rest of the roster looks like and the resources available.
PKMN
Pikachu
Charizard
Lucario
Greninja
Mewtwo
Garchomp
Gardevoir
Blaziken (Echo of C. Falcon)
Overall this is probably the hardest one. Among the other things considered: some sort of thing involving Eevee and/or its evolutions, Gengar, Decidueye, Mimikyu. SwSh I just find it hard to pick a rep that doesn't feel like it's just there to fill the quota; the selection just doesn't quite do it IMO.
FIRE EMBLEM
Marth
Robin
Byleth
Lyn
Ike
I feel pretty strongly about these five, except for having mixed feelings about keeping Byleth over Edelgard. If resources are too scarce for five full uniques, then I guess Awakening could be repped with Lucina as an Echo of Marth (or I guess Chrom if that would still be less work than Robin). In any case, I think it justifiable to just keep one of the Awakening trio; even though Echoes are easy, not including the FE ones could maybe allow for other franchises with less reps to possibly get an Echo character of their own. But, that's right, I think FE can be done well with zero clones.
I think this is mostly reasonable and, even with the cuts, superior to what we've had before. And in a 70-character roster it still leaves over 40 slots for other characters, which at least from a brief look-over I think is just enough. In terms of percentage of the roster, the four series combined would be around what we have now, actually, although the specific numbers vary depending on how you choose to deal with the Echoes. This does skew less towards clones in general, though, which hopefully doesn't hamper the "reasonableness" too much despite this mostly being pure hypothetical regardless.
But, haha, after writing this it looks less like a reboot and more like a proper sequel to 4 than anything else (which is basically what a 70-character "reboot" would be, anyway).