Unova seems like its going to wind up being shorter than Hoenn/Sinnoh. I mean in Japan Ash has 4 badges already at only 55 episodes....that's literally halfway.No point watching it if they change the characters every 30 eps.
Unless you really like Ash I mean.
I suppose though they will keep Iris/Cilan around after the Unova league for a possible filler arc. I honestly can't see who would replace them, as they definitely aren't going to bring Brock back or one of those older female characters.