I do hope that the Gym Rematch system is included in ORAS, it was one of the most exciting parts of Emerald, and on top of that, it really fleshed out the characters of the Gym Leaders, something that, in my opinion, X & Y really failed at.
I don't think that it will receive a perma weather boost, but at the same time, I can see why it's possible, because if normal Groudon has Drought that lasts for a set time, then Primal Groudon would have to rely on that time period if it's ability strayed from a weather setting ability. But at the same time, if it received Drought again, then that would be incredibly redundant. It is likely that their abilities will change and get a sweet upgrade, and it's also heavily implied that they could be new signature abilities, being that they are "powers hidden within", but at this point in the game it's just too soon for any serious conclusions.