The characters never explore spin-off regions since the main series games are often the priority on an advertising spectrum. Game Freak wants to make money, which explains why they only travel to main series regions.
Also, I do agree with XanderO that I could see Ash returning to Kanto since there is not much to do until the possible 6th Generation is revealed. Although a Team Plasma arc can be a possibility since the March 2011 earthquake and tsunami in Japan caused the Team Rocket vs. Team Plasma episodes to be cancelled.