I've always thought that the door had something to do with Volcanion, that we were supposed to one day get a key item via mystery gift that would unlock that door and Volcanion would be behind it.
But they seemed to be in a hurry to move away from XY, so nothing ever happened. Same with Eternal Floette. Probably meant to release it, decided not to, and at this point why bother. The only chance we'll have to see something happen with the door or AZ's Floette would be with eventual XY remakes, and even then it's not a guarantee.