I haven't played Super Mario RPG (yet, I have my SNES Classic and I'll get to it eventually), but I've seen people want Geno in Smash since before Brawl, so I know he's one of the biggest and longest requests next to Daisy and Ridley. I would like to see him, especially so that that request gets fulfilled at last, and because we've seen Mega Man, Cloud, Daisy, and Ridley all finally show up, Geno seems very likely. Definitely above 50% in my books.
As for his third party status, I said this in the Newcomer Speculation Discussion, which is my reasoning as to why classifying Geno and some other characters as third party in the same way as Snake and Cloud is a bit disingenuous:
"I consider Simon Belmont, Geno, and maybe Banjo Kazooie "soft 3rd party characters." That is, they aren't owned by Nintendo, but they exist exclusively or almost exclusively on Nintendo consoles. In contrast to what you might call "hard 3rd party characters" like Cloud and Snake, both of whom are more tied to the Playstation and have little to do with Nintendo.
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Geno's even more extreme in that regard, only appearing in an SNES Mario spinoff and nothing else besides a tiny cameo in the original Mario & Luigi: Superstar Saga, as well as the Mii Costume in Smash 4. And as I said, I believe that the Mii Costume is a sign that the demand for the character is acknowledged, but of the two Square characters, Cloud was the priority, for obvious reasons. Now that Cloud is done and returning, Geno's time to shine is now."