I would love to see Ash replaced, the whole "enters a new region -> acts like a total noob" deal is getting very tiresome, having some new blood in the show would be a very welcomed change. Plus it'd finally let Ash win a league or two which would be a wonderful way to end his focus in the show and he could still come back in the future for supporting roles like his past companions all have so I'd love to see this happen. They'd have to keep at least one character I reckon, otherwise the story just loses all continuity from previous seasons and after five generations that's a bit risky, but there's no reason it has to be Ash. Perhaps have Cilan travel with Iris to the new region before they split ways and the story focusses on one of the two with their new friends? At any rate I don't mind how it happens, I just want to see it haha.