Outside of Red and Blue part of me wonders if Game Freak will be lazy with the Battle Tree and we'll mainly see characters from Gen VI who already have more or less workable models somewhat (with minor alterations) and of course Cynthia, who will probably be re-used for the Diamond and Pearl remakes.
I do recall The Riddler posting a picture of Anabel, most people assumed it wasn't meant to literally signal her return and was a, well, riddle---popular theories include being able to talk to Pokemon, since she had that power, and an Alola Alakazam that was Physical, since her Alakazam focused on the Elemental Punches which are no longer viable post-Gen IV, but those don't seem to be the case so I wonder if Game Freak will "make up" for the lack of the Battle Frontier by having the Frontier Brains appear in the Battle Tree.
I really hope they go crazy with it BW2-style but I'm not getting my hopes up.... the Frontier Brains would be a great inclusion.