THEORY (In other words, everything here may be right, or wrong. What fun. If you don't agree, feel free to state your opinion.)
If you keep up with the main Super Smash Bros Ultimate web page, you probably are aware of the weekly music samples. (I'm still enjoying that Galaga one.)
Anyway, I noticed that after every newcomer is announced, a song from their franchise is quickly added to the slot. (Example: Gankplank Galleon was added to the list the week after the Smash Direct.) Of course, Dark Samus didn't, and with that some might shred my theory right there, but remember that Metroid already got Ridley, and a Metroid song. So there.
Anyway, it seems that franchises that are getting songs are less likely to get newcomers. (Which means my theory says no Kirby. Bother. Oh well, the Geno fans are likely to hate this as well, so I'm not alone.) And yes, this does mean I'm challenging all those Ken from Street Fighter and Incineroar leaks. (I may still be wrong, of course, but I'm just saying, this theory goes against those leaks. Be warned.)
Therefore, these are still the franchises still open for newcomers, according to my theory, and not accounting for the obvious factor of new franchises:
Star Fox: I don't know. Slippy and Peppy aren't the fighting types, and Krystal's already eliminated.
Earthbound (Call it Mother if you want.): I don't know my characters for this one. Moving on...
Game and Watch: No way this is happening. Next!
Kid Icarus: Possible. I don't know my stuff, though. Onwards.
WarioWare: WAA! I'm gonna go out on a limb and say, "unlikely, but maybe." I know, Ashley's out. However, there are still loads of other characters available, and some could fight, like Kat and Ana.
R.O.B.: Forget it. Not happening.
Punch Out: Not likely.
Wii Fit: Who would it be? The Balance Board?
Xenoblade: Oh no. *Hides from Rex fans.*
Duck Hunt: No.
Sonic: Shadow still hasn't been confirmed for an assist trophy. The Shadow hopes still stand strong.
Pac-Man (If you don't count the Galaga Medley.): Mrs. Pac-Man could be quite interesting. She had some unique abilities in her game.
Final Fantasy: I don't know much about this. Maybe that irritating Ninja who's always stealing my trophies in Mario Sports Mix. That guy should get disqualified.
Bayonetta: NOOOOOOOOOOOOOOOOOOOOOOO! No Bayonetta characters! Please!
Mii (If you count it as a franchise.): I don't think so. *Sits in the corner holding a sign saying that Dark Lord's a better boss than the other characters.*
That's it. Feel free to state your opinion.