Ura
You forgot the Miis as they should normally count as a new franchise, too. But besides that, that's what I also grow to think more of. Since there's that pretty small gap between the two Smash entries and most million sellers already included in the game besides Arms obviously, Golden Sun, Rhythm Heaven, Excite, and some casual games like Style Savvy, there's not many options for characters this time.
That's probably a good reason for bringing back smaller franchises in Smash which would make sense. The Switch is literally getting everything they have, there's only Animal Crossing as major IP left and some smaller things like Wario or Star Fox. Due to the enormous Switch success and the will to have a diverse offer of different games, Nintendo may dig out smaller franchises and what would be better pushing for them than an inclusion in Smash?
There's those you have named Ura, but also Chibi Robo, Custom Robo, Project Zero, Sin & Punishment, Dillon or Fossil Fighters.
As for retro fighters, there's also Mach Rider, Balloon Fighter and Mike Jones as bigger names left.
To be sincere, I would love any of them being playable. They just excite me a lot more than another Mario, Pokemon or Fire Emblem rep.