If I had to make an early guess...
I'd say the next pass will be half Nintendo properties and half third party IPs.
I say this because licensing the rights for these characters, as well as their stages, music, etc., must take some of the profits out of DLC. At the same time, Nintendo seems happy to oblige as they have sold an absurd amount of copies of this game, and had to get people to buy into the Fighters Pass.
For round two, however, A) you have fewer people buying DLC as that's the natural progression of a game's lifespan, and B) the people left playing the game and paying for content are more likely to buy the Fighters Pass regardless of who's in it, in terms of percentage of active players. This is important because it means that Nintendo can rely on properties they themselves own, and don't have to split earnings with anyone else as a result.
They've had a year and a half of social media to gauge and analyze, and try to figure out which of their IPs are genuinely most wanted-- not necessarily by small but vocal fanbases, but by large swathes of people based on social media algorithms and internal findings.
I think they start off with a big third party unexpected character, then follow up with a much-desired Nintendo character-- Waluigi, perhaps. Maybe Skullkid. Who knows. But then that will throw all our speculation into a tailspin as we'll no longer heavily rely on third party choices, and Nintendo properties will become fair game again.