The way I see it you can do one of these four things:
Do Ultimate again but bigger or port Ultimate in GAAS style situation like Fortnite, Destiny 1+2, WOW, or OverWatch to the next Nintendo console
I don't think this approach is sustainable long term. See Pokemon after Gen 5 continually having issues with its large roster (and it's arguably more complicated to remake a fighting game roster every dev cycle than a Pokemon roster). Doing this now would effectively just be kicking the can down the road for some other poor sap to clean up.
Soft reboot (IE MK9 approach)
I'm skeptical Sakurai or whoever replaces him can add enough new characters and content to really justify this approach. Brawl and For 3DS/Wii U added roughly 18 -19 characters at launch. If we assume the original 8 (Mario, Yoshi, DK, Link, Samus, Kirby, Fox, and Pikachu) and Captain Falcon+Ness+Luigi (no way they get leave the roster people would riot if any of them were cut or made DLC) are in with revamped movesets that gives you 8-9 slots of dev resources. If we are being generous and say that not having a Subspace or 3DS version to worry about increases newcomer production capabilities slightly we can maybe bump this to let's say 16-18 true newcomers. I'll also be generous and say Nintendo doesn't request any synergy with like Arms 2: Electric Boogaloo or Splatoon 4 with revamped Min Min, Inkling, or whatever. That could work but you'll either need to have a meaty single-player and scant newcomer #s or a decent amount of newcomers and a ton of simpler, smaller multiplayer modes. The former could work from a marketing perspective going off Brawl but the latter might be a hard sell when a lot of the casual appeal of Smash is its pick-up and play nature which usually means just having a favorite house ruleset and sticking to it. Adding say a tag battle/assist mode ala MVC or Tekken Tag Tournament might be nice for the competitive folks but will the casuals that make up the majority of sales really see that as valuable? As an example, I feel like people actively make fun of Squad Strike in Ultimate for how niche it is. Especially when we could conceivably have Ultimate + Switch as a discounted bundle option for the thrifty customer by the time this is out.
Traditional Smash Sequel Approach (normal amount of cuts and additions, few revamps, and one or two big new modes)
This is likely the route I see them taking to minimize the roster equivalent of sticker shock and to avoid alienating casual veteran players with moveset revamps. I expect most of the characters that have easy-to-make echos and semi-clones to stay in this scenario to bump up roster #s to minimize cognitive dissonance as much as possible. This is also the safe and boring approach IMO.
"Smash With a Twist" (Smash in 3D ala Power Stone, Smash with other media characters, Smash Warriors, etc.)
I would like this approach the most but I also think it's the least likely. Changing the roster too much and you alienate casual players and at that point your economically better off just making a new crossover IP.
