Unless I've missed something, I don't think they have a confirmed place for the Grand Festival in Kanto. Where do you think it should be? I think it should be in Viridian City.
Celadon would be my choice. That way Erika could make a (well-deserved) cameo and maybe, just maybe we'll get a joke about how Ash turned into Ashley to get her badge.
^ Use that thread for any Grand Festival discussion. Seriously, there's no need for multiple threads about what's the same thing. Sure, it's more of a thread to discuss what characters might appear, but all discussion might as well as taken there.