I know there has been a lot of speculation, and my thinking is that there has to be a Battle Frontier, or Battle Resort, whatever you want to call it, but basically a post-game that provides fun alternative ways of battling. X and Y introduced Inverse battling after all, and to me it's just so obvious and easy to make a facility out of it that it would almost be bizarre if they didn't.
So given that by all accounts the games as shipped don't have them in yet, so I'm thinking that there might be creedance to the idea that they'll be DLC, but more likely than that, I just think the Frontier isn't finished yet and that they'll patch it in when it's done. It could also be possible that it's part of a promotion event too not unlike the Eon Ticket. Maybe that's just incredibly wishful thinking from me as I know a lot of other people are assuming that it'll not be game like the original Ruby and Sapphire, but then it'll be included in a follow-up Delta Emerald. My thinking though is that in all remake games there has been a good deal of extra content and I can't imagine that the games will play out very similar to the original R/S but then just tack on some more legendaries, some looker missions and be done with it.