Dunno why, but if Geno doesnt return as a Mii costume at all, Im going to half bank on the idea that Sakurai is 'Pirahna-Planting' him. Granted, several sega costumes didnt return with Joker (Akira, etc), but Geno was THE standout Mii costume, with a Titlecard and everything
With how much Geno gets referenced by Sakurai as a missed opportunity (several in the last 2 smash games), I have a feeling he's going to release the character post-fighters pass without a stage but with a few reworked songs like Forest Maze.
The crazy thing with Geno is, he's nearly has much requested as Banjo (along with characters like Isaac) in the ancient fanbase lol. Yes, Banjo got in likely due to being a major character in a once famous Nintendo franchise along with fan requests, but Geno has also been made famous by the smash creator himself, and that to me is one of the most important aspects of him getting in.
Do i think he's one of the remaining fighters pass characters? No. Do i think he's coming to Ultimate somehow? Yes. Just a feeling this time. Its the only time where he probably could, considering this game has smashed expectations on the character front.
If not, i expect to see him and Isaac and slew of other fan created characters via mods years down the line. The game has everyone, and to add to that is the motivation for a lot of people