A quote from the USGamer article:
Trainer customization isn't the only feature that Pokémon fans want to see in Omega Ruby and Alpha Sapphire. The Battle Frontier, which features a variety of advanced challenges in a theme park-like setting, is another popular feature that has lately been absent from the series. Will it also be missing in Omega Ruby and Alpha Sapphire?
When I asked Ohmori, he seemed to consider his words carefully: "All I can really say is that we have quite a lot of content for after players finish the game this time around. We also have a lot of features like contests and secret bases, so right now we're only really talking about those."
I was giving it some thought. I can see reasons why GF would prefer not to tell us the Frontier or some upgraded version of it will be in ORAS - they don't want to spoil it.
On the other hand, if it wouldn't be there, why wouldn't they just tell us? Why would they deliberately give us hope only to disappoint us upon release? This doesn't sound smart. I don't know, if there weren't going to be a frontier, they would've probably broken it to us now, just like they did with customisation.
So maybe there will be a frontier after all.