If a Unova capture really is next for Ash, I can't see it being anything but Zoroark. There's not a lot of other "iconic" mons from Unova left to speak of.
I worry it'll be a bit too similar to Riolu/Lucario, being a two stage canine Pokémon, but maybe that's just me.
But if we're getting one from some other region, I've gotta agree with the Espeon speculation I've seen floating around recently. My personal reasoning is this...
People have always wanted Ash to catch an Eevee of his own, so it's not too unlikely he'll finally be getting one this series - but I feel like keeping it unevolved on a team like this would be an odd choice. Most other Eeveelutions have already been ruled out, but I don't think Espeon has had a real major appearance in the main series in almost 19 years.
Its Pokédex entries say it's "extremely loyal to any trainer it considers to be worthy", and I feel like that ties in pretty well with Ash's character and the bond he has with Pokémon. It'd be his first Psychic-type, too (excluding Mimey, I guess). On more a minor note, it would also have some interesting parallels, seeing as his counterpart Red has an Espeon in GSC, and his very first rival Gary ended up with an Umbreon.
But what do I know, maybe Gou is going to end up catching eight Eevee just to evolve them all...