Well... the filler saga was expected... they wouldn't stop anime to wait for X&Y and they couldn't really do X&Y saga right now, which is about half a year before the actual release.
I do hope it will be Orange Isle/Battle Frontier -like saga so we can see Satoshi finally winning something again...
Well... according to timeline it should happen... Indigo - Orange (filler, won) - Johto - nothing - Hoenn - BF (filler, won) - Sinnoh - nothing - Isshu - new saga (filler, ???)
Since this follows a pattern, I do hope it's a little league with Satoshi's win at the end.
About Miju's leaving Satoshi... Nah, I don't think that's the case, since the episode is called "Road to become king" Miju will probably want to be king, Satoshi will help him but Miju will soon be bored and decides to leave with Satoshi.
I don't think this is Bulbasaur's-like episode of Indigo League but more like Squirtle episode where the party reached the island with Blastoise and Wartortles. And Squirtle also only helped his fellow nakama instead of thinking of staying with them.