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though customization was considered one of the most highly demanded features in a pokemon game, i believe gamefreak tries to keep new features to only their release titles per generation. ie: contests in hoenn but not kanto in gen 3.
however, most of the time these features return a gen later (contests gen 4). on top of that, the central theme of the kalos games was meant to be beauty. it's hammered in everywhere you look from the pokemon to the the art style and to the gimmicks. thus it'd be weird to just copy-paste one of the region-defying themes into another game within the same generation. otherwise why would anyone want to play X/Y anymore? look at how much shit was shoved into gen 5 and how NO ONE plays them anymore. as much as it was demanded, i'd honestly believe it will return in gen 7. just hold tight guys. they might give small customization options with some of the later gen 6 games hopefully.
now onto the frontier. i'm pretty sure the battle frontier will either be revealed in next corocoro or kept secret until release. i have a theory that all post-game content is hidden by clouds on the map that we got. spear pillar being one and the battle frontier area being another. both are obscured by clouds and for good reason too. don't give up yet! BELIEVE!