I would like to see Ash go back to Hoenn, Kanto and even Johto again. Johto has so much new to see thanks to HG/SS, and could be home to the Gen 4 Battle Frontier. All three regions could be travelled by Ash as a training thing. He could decide to go to each region again and face their gyms, but being a more experianced trainer he would qualify to take part in the more challenging battles, like the rematches from Emerald and the Saffron City Dojo battles from HG/SS. Gym battles in the anime seem to be three-on-three usually, full six-on-six battles against proper teams would interesting. I'd like to see Ash in a full battle against a more experianced Misty who uses the likes of her Starmie and Gyarados from the anime, and a few new captures, like a Quagsire, Milotic or Lanturn. He could battle Brock's brother Forrest, who uses his Rhyperior and pokemon from Brock's Dojo team; Rampardos, Relicanth, Kabutops etc. A full team of Dragons from Clair, a battle against a full group of Ghosts from Morty, a fist-fight against six Fighting types from Brawly, a full fire team from Flannery, it could make for several interesting series, and could even include returns to Sinnoh and Unova as well.
Such a series, though, might require a travelling companion being tutored by Ash; Leaf could be used for Kanto, for example, and she could battle the weaker teams that Ash once fought.