I honestly want Ash to still be the protagonist. Him re-visiting regions is interesting to me. (Battle Frontier is probably my favourite season, in part for this reason, even if it didn't really go too in-depth with that aspect.)
I just hope it has a linking story arc, and isn't just Pokemon Generations v2 with what are basically self-contained episodes jumping around the world. (Don't get me wrong, I like Generations a lot. Fleshing out the game storylines is awesome. But it doesn't work well for a main series anime, and is more suited to shorts like Generations did it.)
Seeing how the anime characters Ash has interacted with over the years have changed would be cool. He's promised to go back and battle/visit so many characters, it's about time he actually went and did it. Some characters (especially the Kanto Gym Leaders aside from Brock and Misty) sorely lacked character-building, this would be an easy way to fix that. Also gives them the excuse to introduce game characters to the anime canon that are yet to appear in the anime, such as Janine or other protagonist characters.
It would also give them an excuse to have him use his old Pokemon again, which he hasn't done since what, the Unova League? Another reason I liked Battle Frontier, along with the fact he was more competitive in those battles since the opponents were supposed to be tough. (They didn't have to rubber-band his strength like they did in some other regions. Which I'm fine with, obviously him sweeping Kalos's gyms with Pikachu would be super-boring, but seeing him at his best tends to give us his best battles.)
This series will either be amazing or terrible IMO. There's no real in-between. If they handle it right, it will be very cool. And hey, if Ash IS still the protagonist, at least there'd be an excuse to have him go and get back his Pidgeot and Primeape like he said he would.