Yeah, I'd like to think that they do bring it up being that it was like the "closing reason" we haven't seen Paul since the end of DP. With Ash in Unova and in the near future going to the new region. I'd hope Paul had to challenge the ENTIRE Battle Frontier in order to face Brandon again simply to let me accomplish with his brother did which was win all 6 symbols prior to losing to Brandon and he would also accomplish what Ash did and that was to win the ENTIRE Battle Frontier. That would "keep him busy" while Ash was off in other regions.
Then have Paul defeat a league (my guess is re-challenge Sinnoh League and win) in order to face the E4 and then Cynthia. The ONLY reason I'm mentioning this is so that he can enter the Champion League but again I have no idea HOW a trainer is able to enter it my guess is simply defeat the E4 of a region (my guess their home region but I'm not sure about that) and its Champion.