Only two things are for certain.
Multimedia characters would be too hard to license (the only official reason given for them being banned, last I checked. You could also argue it's an aesthetic thing but I wouldn't given that licensed games, like it or not, are a major fixture in gaming). Sony is theoretically possible, but very likely to just say "no" because of their standoffish practices and likely still lingering malice towards Nintendo.
Aside from that, the sky's the limit, at least until we get a solid grasp on what post-pass DLC will actually be like (let's say post-pass 3, or DLC fighter 9).
Right now, I think the Fighter Pass will conclude in one of four ways. The second platformer route (Crash), the NES icon route (Ryu Hayabusa), the Capcom character route (Leon or Phoenix), or the guy who can't get out of the leaker's circle (Doom Slayer/Guy). Something tells me that at least one of Namco's costumes are coming back with Terry. I don't expect any female characters out of this pass.
Speaking of, here's my predictions on Mii Costumes for Terry: Terry's Hat (Headgear), Heihachi, Geese Howard, Akira, Jacky (Brawler), Lloyd Irving (bring back SSB4 costumes) or Yoshimitsu (keep with the Fighting Game theme) (Swordfighter), Nakoruru (Gunner, Mamahaha with beak open is the gun).
- If Fighter 5 is Crash, I could see Tracer, Spyro, Arthas, and some characters associated with Platform Games like Shovel Knight.
- If Fighter 5 is Ryu Hayabusa, it'd be an NES classics pack, like Kunio-kun as a Brawler, or Bill Rizer as a Gunner.
- If Fighter 5 is Leon or Phoenix, it'd just be a Capcom avalanche of everything still missing. If that's not enough, a Frank West Mii Costume could finish it, but I dunno what type he'd be.
- If Fighter 5 is the Doom Slayer, it'd probably just be a miscellaneous pack like Banjo's was.
- There's other options for Challenger Pack 5, but I kinda doubt them right now.
After that we have several, several questions. What will the non-pass Fighters come with, if anything? Could new Assist Trophies and Pokéball Pokémon take the place of new modes, even starting with Terry Bogard? Will future updates matter beyond bug fixes, balance changes, and the paid DLC? Are they mostly first or third party? Even the circumstances of their release lie in mystery, though the implication is that they will be handled much like Smash 4's DLC. Then there's how many post-pass are we going to get? It's not just one, otherwise they'd have specified a "one last Fighter" instead of "More Fighters" in general. But it could go from as low as 2, to as high as all slots filled for World of Light, to even flat out triple digit Fighters. I draw the line at 100 Fighters being the absolute highest we'll get. The lowest is still impressive: 77. Are Echo Fighters coming back, or will DLC continue to focus on Fighters that have unique movesets? Do Spirits, (1stP) franchises/(3rdP) companies having a character as DLC, Summons, Boss Fights, Base Game Mii Costumes, DLC Mii Costumes, any of that put a character out of the running? And for Frisk in particular, are they nearly guaranteed or a pipe-dream now that Sans and Megalovania are in, but one is a Mii Costume and the other is in the "Other" section of Music?
There's too many unknowns. I think I've got a fantastic guideline for post-pass DLC, but I have no serious clue as to who I'd bet on being in.