I figure he's pretty likely. Characters like Banjo and Kazooie, Ridley, K. Rool, Simon, Chrom, Inkling, and Isabelle (among others) got in due to sheer fan demand and support - in spite of some people claiming that Geno's support is just a meme (
which it isn't; detractors said the same thing about Ridley's support in the lead-up to Ultimate), the main obstacles that people pointed to were spirits not being promoted to playable in the first Fighters Pass, and a lack of relevance. By now, it should be pretty clear that neither of those points are obstacles to a character getting in.
Geno's in a weird place as far as characters in general are concerned, being from a first party Nintendo franchise but being owned by a third party company. (which doesn't seem common - it's not like Capcom owns Vaati from Zelda: Four Swords or Veran from Zelda: Oracle of Ages, as far as I know) Obviously, he wasn't going to make it in before either of Square-Enix's flagship franchises, but with both Dragon Quest and Final Fantasy in the game, and his fifteen-plus years of dedicated Smash support, it definitely seems like his chances are better than they've ever been.