I agree with Xanders points. By all indications we have seen he should go to Kanto. Heck even if the intro they have Ash pokemon at Oak Lab. So reunite him with them and they embark on some Filler Arc.
For me though it all fringes on what the League results are. If he wins than I see a Champions League, if he loses than I see some random arc that occupies us till 6th gen. And yes it can happen soon, there is nothing to stop it from happening. So after Movie 16 or even a little before I wouldn't be surprised if we see or hear of some 6th Gen related things or pokemon. There could always been a Plasma interruption which can lead to a Plasma arc, which gives them a little more room to develop Ash pokemon he has with him.