I'm going to wait for ZA to come out before I make any decisions relating to it, it's going to be tricky mixing in content from that and Gen 6 together considering ZA is after a time-skip.
I DO have plans for the MC to go on a tour of the regions once the story is over and he's on his way back home, that would probably be the best time for ZA content when he gets back to Kalos, so it fits your idea pretty well