This is something i've never thought of... Paul has already participated in the Kanto, Johto and Hoenn leagues. Once Ash knocks him out of the Sinnoh league, where's he going to go if not to Isshu? The Battle Frontier or something? I guess if he was to return to Kanto to do so, then another battle between him and Ash could occur quite easily.
I could quite easily imagine Ash being at home with his Pokemon after the Sinnoh league and he see's Paul walk past, thus resulting in a final quick battle at the ranch. That would be interesting.
Well the deleted blog also said that Isshu is going to have a character similar to Paul, hinting that there will be no more Paul. The only option would be the Battle Frontier. But for all we know he will challenge the Sinnoh BF.