I may have been a little unclear in my theory. I truly don't think Nintendo would ask Sakurai to add Geno into the game, as it goes against pretty much all of the standards they've set for the Mario series right now. As of now, we don't know the extent of the "Purity Team"'s influence on the series as a whole, but I can assure you that we'll never see most of the new characters in Odyssey ever again. Mario as a series nowadays may have a few new characters introduced in every game, but they can't be too divergent from how the other characters are and they can only be in one game. Geno being in Smash would pretty much solidify him as a part of gaming history, and I don't believe Nintendo would want that as it goes against what they have recently established for the series by making a random, one-off character important to the series instead of giving the spotlight to the main Mario cast. It would also set a precedent for side characters to appear in more games than their initial appearance and take the focus away from Mario, Luigi, Peach, etc. And since every DLC choice has been made by Nintendo, Geno's inclusion would seem to only come about if Sakurai and/or Square Enix requested it and is able to convince Nintendo to do it.
Also, keep in mind that I'm not trying to hate on Geno or say that he isn't happening, but that I'm just trying to give another perspective on Geno's inclusion. I would absolutely love to be wrong. I just feel like speculation is only healthy when you consider every possible outcome, and not just get tunnel vision on what you personally want. I'm not claiming to know exactly how Nintendo or the "Purity Team" functions, I'm just making inferences based on how the Mario franchise has evolved over the years. Because of this, I personally believe that Nintendo's current attitude about how the Mario IP is represented is Geno's greatest barrier of entry right now.