So i've heard a lot of theories about how people think Korrina will be the champion in the next kalos game. Here's why i think this wont be the case.

First of all the past gym leaders who made it to the championship were the strongest gym leaders in the region
Wallace was the final gym Leader in RS then the champion in Emerald
Iris was the final gym leader in White then the champion in BW2
Blue who was the champion in RGBY later on received the role of the final gym leader in GSC

Korrina is "just" a third leader.
On top of that she already plays two roles in XY (gym leader and Mega Evo successor) we even battle her 3 times. Giving her the third role would be really out of place especially since other gym leaders are so underdeveloped

Not to mention that Korrina as a champion would be basically Iris 2.0 and GF dont like to repeat themselves. So far every champions were very different and unique.

Besides there's no sense to switch Diantha when she's also pretty underdeveloped. So far the champions who were replaced (Steven and Alder) got a pretty prominent role in their games (RS and BW). Diantha is more like Cynthia who also had a tiny role in DP but got a much more to do in Platinum. Diantha needs the same treatment