I'm pretty sure this has already been talked about, but... If we get a stage with Geno, wich one do you think is the more likely?
Iv'e been thinking about the more obvious Forest Maze and Star Hill. But what would be the best stage for me, is the top of the Bowser Castle, with Exor in the background. He could be a boss like Metal Face, and is to me, the only spoiler-free boss they could add. (Yeah SMRPG is 22 years old, but I don't think Sakurai could add Smithy as a boss, since you only meet him at the end of the game, the anticipation for him is what make him a cool boss in my opinion. If he's in a stage, I think it would kill the surprise. Could still be a trophy tho.) and if the players manage to beat Exor, they'll be taken to Smithy's Dimension with a different layout and stuff.
Also, a stage like that would be more suited for more epic music. You could say that we have Animal Crossing stages, and even Hannenbow, but I would just prefer my idea over something like Forest Maze.
Or maybe we're not going to get a stage, but it seem unlikely to me. Geno is too different from the others Mario rep to be like Bowser Junior, and fit in in Rainbow Ride or something like that.