Sephiroth got in likely due to high fan demand, and in terms of villains of third party characters, I feel Eggman is next in line--Gruntilda barely registered on previous polls, though with B&K's appearance probably has more demand now. So while Gruntilda doesn't have to be playable, it was a missed opportunity to not have her as a boss fight, due to being DLC. I feel like if Banjo & Kazooie came back, at the very least, she should be a boss fight. One of the best boss battles in platforming games.
And as for Geno, anyone saying a character is disconfirmed has obviously not been paying attention to fan rules being shattered left and right. That being said, if we were to talk about likelihoods, I feel like Geno and Sora are much more unlikely now. Which I'd hate to say it, because Sora would have been a truly WTF character, and would have brought a whole new game world, unlike Sephiroth. Geno's mii costume not returning makes a lot more sense if they were waiting for Sephiroth's release to reveal it--which is terrible if that's the case, because such a highly requested character should be shown off asap, rather than mid-point of a second pass. Hoping it's not the case, I would love both Geno and Sora to be in the pass, but we also have to prepare ourselves so we're not too disappointed.