It seems pretty clear that characters get added for more than fan demand. Otherwise we'd have never gotten Snake, Bayonetta, Joker, Terry, or Byleth, among others.
I think there are certain aspects of DLC characters that might play a role going forward, but it's hard to say if they will hold true right now. If Geno is added based on fan demand, then he'd be the first "non-protagonist" to be added through DLC. Banjo was the first western origin pick, but is he going to stay an exception or will we get somebody like Crash or Tracer? Does the circumstantial evidence around Dante lead to him as a playable character in Smash, or does it mean nothing? Will another pick be an "upgrade" or are we already done?
Right now, we don't really have any answers. I would like to think that the next fighter will answer SOMETHING, but I'll close with a few ideas.
1) I would suspect we're getting at least one "new" company, whether it's ArcSystemworks or Koei Tecmo, or even another like Level 5 or Marvelous
2) There'll probably be AT LEAST one "hype" or "fan demand" character
3) There'll be AT LEAST TWO 3rd parties
Beyond that, I don't think there's much we can go on. If we get a reveal in the next Direct, and that Direct is tomorrow, the one character that makes "marketing sense" to me, if it's 1st party, is Paper Mario. Game just came out within a week, would still be able to impact sales of the game. If it's 3rd party, my gut says that Dante is in this pass but I don't really know who would be revealed as Fighter 7.
Either way, go into tomorrow with an open mind. Even if it's somebody you didn't want, you might try them out and absolutely LOVE them.