About whether or not Geno would be a Mii Costume in Smash Ultimate:
A big majority of Smash Ultimate's newcomers have been a Boss, an Assist Trophy, a Mii Costume or any combination of the three in Smash 4, with King K. Rool, Isabelle, Dark Samus, and Ridley making the transfer to playable. I get that some mention of some characters not in Smash 4 have been made, like with Snake, but considering the upgrades transitioning from the last game to this game, there's still a bit going. I'd want this tradition to keep up, tbh.