So I will respond to this one briefly because it isn't just simple dismissal of his chances for no reason and is an actual talking point.
See, even
@papagenos has taken this stance of thinking that Geno is possible after this returning Mii Costume is some sort of hypocrisy, but it really isn't.
There is a difference between saying 'Returning Mii Costumes deconfirm, but not Geno's because we want him' and saying 'Returning Mii Costumes don't deconfirm, but some of these characters have other things against them, and Geno doesn't' and I really think everyone sees Geno fans who are still rooting for him being either delusional or the former instead of the latter, of which I am a part of.
When new information comes in, one is allowed to take that information in and come to a new conclusion, and the results of that conclusion may or may not differ. The conclusion I've come to was that I previously thought that since some costumes had already not returned that some would just be cut, however seeing the Chocobo Hat return with Geno and that they have clearly made the effort for each third party Mii Costume to return from Smash 4 over time, this means it was always part of the ideal and contracted for them to return and therefore has no weight against them being playable.
This means that I open the window for characters like Tails, Heihachi and the now inevitable Lloyd Irving and Monster Hunter costumes that WILL return. This also happens to apply to Geno, however it wasn't until the Chocobo Hat returned with Geno's costume that people such as myself realized we had been living by another stupid fan rule. It's the 'Spirits Deconfirm' fan rule all over again.
Frankly, the only costumes you should maybe worry about are new ones, ones exclusive to buying a Fighter's Pass, or Deluxe/Premiums, and that is the order of severity. New costumes are the least to worry about because honestly, as long as content keeps coming, these things can change in the future so long as fan demand stays strong. Remember, Spring Man was legitimately considered by Sakurai to be that playable character for ARMS despite being s Spirit, costume and an AT. So, costumes like Travis would fall here.
Things like Rex and his costume is a bit rougher because they are technically more expensive and locked behind purchasing other content, but I wouldn't rule them out entirely.
However, Deluxe/Premium costumes are a whole different ball game. These are painstakingly made to be as accurate to the character as possible and occasionally even come with music tracks when applicable, and thus so much effort being put into them to be like the character really feels like there are no intents for that character to be made playable. Thus characters like Sans, Cuphead, Vault Boy and Bomberman are kinda screwed in comparison.