I think it's because A: Geno is popular, B: Sakurai apparently really likes Geno and thinks he'd be a fun and unique addition, and
actually wanted to get him in Brawl; it didn't work out, and he most likely considered him for Smash 4 (if it weren't for, you know, Cloud, Sakurai probably would have tried to negotiate Geno as more than a Mii costume). So, with the fact that Sakurai has been planning this game in advance for a while, it's very likely that he at least brought up geno when renewing his license for Cloud, because of the fact he wants him in Smash and he's a popular guy.
This does not make him a lock, of course, and I still won't hold my breath for safety reasons, but it does definitely make him all the more likely, especially considering the fact that the roster's most likely designed around fanservice.