Let's try something new.
WITHOUT REGARDS TO SMASH AND REPRESENTATION...
How successful you say each franchise has been since Ultimate's base game? To give you a couple of examples...
Metroid: Metroid has continued the momentum started by Samus Returns and the Prime 4 reveal and has gained a new landmark title in Metroid Dread. Metroid Dread is now the best selling Metroid game. Prime 4 is still on the horizon. The franchise is more successful than it was previously.
Star Fox: Star Fox has gotten references in new titles and Star Fox 2 now has easy access due to being added to SNES online. Star Fox has had a major tie in with Star Link and was marketed heavily for the game. The franchise has not had a new game since continuing the dry spell only interrupted by Star Fox Zero.
I'd like to do more, but I want to see what you guys think. I also don't have a ton of time.
I will do the DLC ones real quick.

: Persona has been eating well enough. Persona 4 Golden came to PC and did very well. Royal came out and honestly is the definitive way to play Persona 5. Persona 5, Persona 4, and Persona 3 Portable also are all coming to Switch and Xbox now, which is also great.

: DQXI seems to have been the biggest game that DQ has had in the west, and I assume it did well in Japan. DQXII has been announced and the series has had a healthy dose of spin offs.

: Despite nothing new coming his way, I would still say the past few years have been a net positive for Banjo given how low the bar has been. A healthy dose of new merch, Rare Replay is on Game pass, and Banjo Kazooie is on the N64 expansion pass. Given the dire straits Banjo had been in for a while, this was a good past couple of years.

: I'm gonna lump KOF in here since Terry is there.
There is a new Fatal Fury/Gaoru game in the works though! KOF XV did well compared to past games, which is nice.

: ARMS has not gotten anything since its initial wave of post launch content.

: Minecraft became the biggest game of all time. It also has had a healthy slew of updates over the years and a few spin offs release.

: Tekken 8 was announced for the PS5, Xbox Series X, and PC. Release date unknown at the time but it is clear more Tekken is coming.

: KH3 came out and did well, as did its DLC in Re:Mind. KH got ports to the PC and Switch, though I would dodge those since they are Cloud ports. KH Melody of Memory came out as well. KH 4 is also in development, though there is no release date.
I have awakened from my hibernation with another roster for a hypothetical Smash Ultimate Deluxe. Characters with red borders are base game newcomers, characters with green borders are echoes, and characters with yellow borders are DLC fighters. The unmarked Pokemon is a placeholder for a Gen 9 (or 10) Pokemon, while the unmarked newcomers are placeholders for whatever is being promoted at that time.
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Pretty solid roster at the end of the day.
One thing I am a little iffy on is the idea of X and Zero as a swapping character. This is not for the reason of not wanting them. I think both are super cool characters and I would love for either of them to be in Smash. I just feel that them swapping would feel a little weird? I'd rather see just an X moveset or just a Zero moveset personally. I think that it would make more sense for them to be separate, since they are far more often apart then they are together. I just feel if we were to get a character from the X series (which imo is a decent take post Sephiroth), I think it would be one or the other if that makes sense.
The only other miss imo is the lack of Astral Chain on the roster. I think with Nintendo purchasing full rights to the IP from Platinum, it is clear they have plans in store for the franchise. It also is one of the big franchises from the Switch era that are not in Smash. It did not sell gangbusters, but it did sell pretty well at the end of the day. I think with its solid sales and Nintendo buying the series it would deserve a look, but the fact you would get a super unique character feels like gravy to me.