Franchises tend to fall into different tiers, with the Nintendo heavy hitters getting 1-2 new characters in every game, mid-sized franchises having 2-3 characters total (with Zelda and Metroid being a bit higher), and smaller franchises staying at a single fighter. So far 3rd party franchises are maxed out at 2 characters per franchise, and 3 total per company. I also looked at my earlier list of which fighters are being requested by fans, and I looked at which big franchises are not in the game yet.
Considering the trends, Nintendo first party franchises don't really need any more characters. Metroid just got two new characters, and Zelda got revamps of its leads plus a couple of new Mii costumes. Ring Fit and Dragalia Lost didn't really get popular until after the DLC was finalized, so they seem unlikely, and I don't think they would add a small franchise or retro pick as paid DLC. If they were going to add a Nintendo character, Pokémon and Splatoon seem like the most likely options since they each got one fighter in Ultimate.
If not for the consistent distribution of franchises/companies so far, I would say that Capcom, Sega, and Square have a lot of obvious franchises for the last slot. With things as they are now though, I think it's likely that they'll leave those for the next game. SNK won't get a new character either, but Microsoft or Namco could. I could definitely see Doom (toned down a bit), Dark Souls, Tales, or Katamari Damacy (if Sakurai came up with a concept that worked for a fighter) being a the last one.
The last option is a new 3rd party for the series, or even an indie franchise. There are both a ton of options here, and not that many that seem super likely. The top ones I came up with are Crash, Rayman, Fortnite, Overwatch, Ninja Gaiden, Mortal Kombat, Cave Story, Touhou, and Hollow Knight. I feel like every one of these would be fine as a Mii Costume. Fortnite is easily the most popular, and that main character looks bland as hell. I didn't think Minecraft would be a good addition either though, so who knows.