Im not sure if this will be a Gen 7 game or the end to Gen 6. But Im putting my bet on Gen 7. They could be doing to Zygarde what the did to Meloetta, just making him a part of the show and not a movie. Cause we haven't seen him for the new movie yet. All we know is it has Volcanion and the new Magearna. They could very well be skipping over him, trying something new. Then again they could have Zygarde as a part of these new games as a Gen 6.5 game. Though I doubt it, the feel I'm getting from these new titles is very different than the XY titles.
And with them breaking the pattern in Gen 5, with 2 direct sequels instead of the usual third version, it wouldn't surprise me if they went this route of not having a Pokemon Z/XZ/YZ.
On a side note, I think these games is where we will get Lugia and Ho-Oh mega evolutions/Primal Reversions. Lugia represents the sea (which is related to the moon) and Ho-Oh represents the sky (related to the sun). They won't be the main focus of the games, more like how Mewtwo was in XY, just a bonus. But I also feel every pokemon related to the sun or moon will get a Mega Evolution in these new games. i.e. Volcarona, Solrock, Lunatone, Cresselia, and Darkrai etc.