Following on what
Malo Mart
mentioned in regards to potential Mii Costumes coming alongside Sephiroth, I do believe we might be looking at 4 - 6 Final Fantasy-specific Mii Costumes (maybe three for FF VII, and the others from other FF games). The Chocobo Hat already takes one spot, and certainly they'll be adding the likes of Barret, Tifa, and maybe Aerith as well (most likely their original polygonal forms). The other Mii Costumes could include as some have mentioned before the likes of a Moogle, Black Mage, Tonberry, or heck maybe some FF protags (Cecil from FF IV, a Clavat from FF: CC), etc. Now, as to who could be a premium Mii Costume in this wave, I think this might be another one which won't have such a costume. Usually the premium Mii Costumes have been from characters from different franchises (very different from the accompanying Challenger Pack).
Now, from the bits of news after some characters have been revealed and released (such as Steve), and learning that said characters had been in negotiations for years, the case with Cloud, Sephiroth, and potentially much more Final Fantasy content in Ultimate does raise the question of how those negotiations actually went. As some have mentioned, perhaps Cloud's lacking of FF content in base game was in fact due to Square Enix wanting a better deal out of their IPs in the game, and thus conditioned his return with the eventual addition of more FF paid DLC in the form of Sephiroth's Challenger Pack. The inclusion of Hero and DQ content, although also negotiated with Square Enix, getting this IP in particular I believe was more of a promise or task the late Mr. Iwata had asked of Sakurai. I do believe having read this some time ago. To speculate more on the matter of negotiations, there is the possibility that the inclusion of Hero and Sephiroth as paid DLC were conditions set by Square Enix in order for Sakurai to ask for any other additional content, i.e. Geno. At the end of the day is all a matter of how much profit they can make as a Company from licensing their intellectual property for use in a partner's game. Projecting a positive outlook from CP 8 sales might have been the key for Square Enix to give the OK on Geno. Going a bit further on this; in order to accept licensing Geno as a playable character for Ultimate, either CP 8's sales, or the combination of CP 2 and 8, should be more than enough to compensate for potential low sales of a CP containing Geno (or stand-alone Geno fighter DLC).
Thus, what then if Geno does not appear this upcoming Thursday? Well then, chances are that he might just be either (a) Challenger Pack 11 as the Nintendo rep (despite being owned by Square Enix, yet he was originally created solely to fit the Super Mario universe) or, (b) a bonus character post Pass.