Well, because the new region is based on Europe, I think it'd make sense for Cilan to go there. So at the very least, instead of having him go, "Welp, I have to go back to the Gym because of an emergency!" at the end of this season, the writers should actually go to the new region with Ash, but then part ways a few episodes in. Then he can pop-up a few times throughout the region.
If Cilan does stay for the next region, I hope they do something with him to keep him fresh.