A reboot would only be acceptable if they revamped the gameplay imo. Add a 3rd dimension or something. If it's just Ultimate but a tiny roster they can fuck off.
I would personally like to see them go back and overhaul/revamp the old fighters for once. It is pretty obvious how outdated most of the 64-Brawl roster has become as they have refused to change any of them for multiple entries now outside of maybe slight tweaks, at best 1-2 moves. Ganondorf and Wario are the most obvious in really needing an update as even with Ultimate's changes, they still barely resemble themselves. Hell, would argue the entire Mario cast pre Wii U could use an update, all of them feel foreign by todays standards
I could accept it if they brought in lots of new characters, like ditching Mewtwo and Jigglypuff for Meowth and Eevee, and replacing all the Fire Emblem characters with new ones, swap Falco and Wolf for Krystal and General Scales. They could drop the 80+ roster to like 40 and make it still feel pretty fresh, but not if they keep all the same characters, except halved with little new.
Building off above, would like for them to really revamp the old cast while including some new faces. Best outcome would be to create a more evenly divided roster that actually reflects how popular an IP is. Looking at Ultimate, Donkey Kong, Kirby, Zelda, Splatoon and Animal Crossing all feel vastly underrepresented for how big of IPs they all are, meanwhile FE should be halved. Would also like better console representation, as weird as that sounds. For being one of Nintendo's greatest successes, it is surprising that the DS only ever got one character (Lucario). One would think DS could get like a Rythm Heaven rep or one of its many other decently popular new IPS : Style Savvy, the Cing Games, Brain Age, Nintendogs, etc..
Also, think getting a new artstyle would really benefit the series. Ultimate looks like shit in many areas as they are still using models from the Brawl era which look terrible under some of Ultimate's lighting conditions. Most obvious examples I can point to are shirtless Shulk and Path of Radiance Ike looking particularly bad, especially on the results screen. I don't even know how to describe Ultimate's style other than it feels like it is colorful, yet also drained of all color at the same time, they look detailed, yet also look like plastic toys.
I do agree with the Nintendo history but, and better single-player, but a big roster with tons of 3rd party definitely didn't wear me out. If anything I just want more.
The 3rd party just felt overdone after awhile. I think a bigger issue is how the 3rd party feel like guests now, rather than an organic part of the cast.
Using Brawl as an example, Snake and Sonic felt like they belonged as work was put in to organically integrate them. The Subspace allowed them to interact with the cast and in Snake's case, he got the Codec easter egg to really sell him as being a part of the world.
In terms of Ultimate, only Castlevania felt like they put any work in to really add them the same way. Castlevania was cool with the boss, the mini area and general love towards the series.
I don't know what it is about the DLC characters, but all of them feel off, including the characters brought from Wii U to Ultimate. They feel less like characters and more like the newest gimmick to break the game.
IDK, just something about Sonic, Metal Gear, Mega Man, PAC-MAN and Castlevania feel so much better implemented than the rest of the outside franchises that I would honestly just keep them and get rid of the rest if we had to keep 3rd parties.
I also just hate how the emphasis went away from Nintendo characters, especially for DLC. It is a Nintendo crossover, yet only 3, technically 4, of the DLC characters are from Nintendo.