I think it'd be nice if they make this short but not so much. Just right to let the development happen without making it too long. It's unacceptable that this would be longer than Hoenn or Sinnoh since it doesn't even have a sidequest like Contests.
That being said, I think it'd be cool if they go back to Hoenn after this if it will be short. Then Ash's goal will be something related to a new feature added in the rumored Pokemon Z, but placed in Hoenn like they did with the Battle Frontier. Double advertisement then, though this time the remakes came first so it'd be weird being late with the region. Though I don't mind
Serena should so do Contests if they have nothing else good planned for her (I think many if not all could agree that it's better if she could do Contests than nothing) but let her start in the next region. XY could be the build-up for the goal she wants, until in the end we see her finally decide what she wants to do in her life. (Besides travel with Ash)
Who wouldn't want to see the XY gang prolonged? They should make it a tradition that all four-member groups get extended for another region (Well not really since original trio did it with Johto. Sinnoh just broke the trend)
Oh and so many old characters can appear again in Hoenn if they want. Of course May, and maybe Dawn if they want to show her still traveling there. There's also Iris if she did go there for Rayquaza.
So basically to sum my post I want it to be not that short but short enough and sweet so it can get extended.
(sorry if off-topic)