We cannot say with absolute certainty that Ash is even going to Alola, especially when, for all we know, they may just retire Ash simply because of XY's horrible ratings, bad movie sales, and poor merchandising sales (Anime may survive bad ratings, but there's little chance at survival if they also have bad merchandising sales and bad movie sales, and believe me, if one of their movies did so terribly it made even Pokémon Heroes look like a smash hit, you KNOW they did bad), and that's even assuming they're even going to actually HAVE an anime for Sun and Moon at all, instead of just cancelling the anime outright due to the reasons listed above (and it's not like a new game entry automatically guarantees a new anime series: DIC stopped doing Mario cartoons since Super Mario World, and besides, even the Pokémon Anime skipped over some games, like Pokémon Ranger or HGSS/ORAS.).
In the increasingly unlikely event that they do have Ash pick a starter, though... Rowlet will probably be the most likely choice since in the few regions he did go with one starter, Kalos aside, he generally went for the Grass Starter (Treecko and Turtwig for Hoenn and Sinnoh, respectively). That being said, they could start a new pattern by having him stick with the Water starter instead, and even the fire starter would spice things up a bit. Heck, he even went for all starters in Kanto, Johto and Unova, so what's to say he doesn't go for all three, or even two of the three, since Ash got Chimchar and Turtwig, with only Piplup sticking with Dawn?
