Happy Hydreigon
Kahuna Mikah
There is still the thing about the SS Aqua returning though... And if we assume that pokéarth is the same as real earth going to kanto or sinnoh post-game would make sense.
I have a feeling that the lack of gyms in Sun and Moon will lead you to travel back to Kalos(because unfinished plots, etc), probably on the SS Anne, that's where we get the new Mega Evolutions, as well as our fix of Gym Challenges since there are none in main region