I actually think Geno's chances are pretty low. Yes, Sakurai talked about him. In March 2016. After the design document was finalized. I really don't think Sakurai would have talked about him in detail like that if he was going to be included as a playable character in a game he was currently producing.
I am somewhat hopeful in that Golden Sun is easily the most requested new series to get in to Smash. Splatoon already made it, but that's it so far. Usually we get several new franchises with each game (don't have time to tally up the full numbers again right now, but if someone else wants to, it would be welcome). So far we've gotten one. The only other heavily requested one is Rhythm Heaven, thanks to those who jumped on the series after the Gematsu leak, but in the polls, it hasn't had near the polling success that Isaac has (and to anyone who argues that the polls just refer to the hardcore/internet crowd, I'd argue that those asking for Rhythm Heaven are the same group, because they're mostly people who heard about the Gematsu leak, rather than a raw fanbase).
So, that might give Isaac a little more of a boost. I mean, we get Ridley, and Inklings, and we'll probably get K. Rool, and maybe even Dixie, Ashley, or Elma. But they're all from series already in the game. Wouldn't Sakurai want to continue to broaden the pools of games that he's adding? Especially with one that is regularly placing in the Top 5-10 in most polls and hopefully the ballot?