>She's in Sun/Moon because they're weeding out leftover Frontier Brains
>Next return will be Noland or Thorton from Battle Factory
>DPPt will have another "Battle Frontier is being built" tease with a copy-paste Battle Tree in its place
>The generation after will have an amalgamation of the previous Frontiers built in one place that contains all the facilities in a World Tournament esque area.

At least that's my take on it. Anabel is there for a reason. It's no coincidence she appeared in the games that right after the Hoenn remakes, where they teased her Battle Tower but never let us in and meet her.