Like the above posts, a Pokemon Z is not going to happen. In fact, it's never going to happen in my opinion.
Masuda has a current motto of "we are always looking to surprise people" and if a Z were to come out, the audience would have expected that. Same with Gen 5 and a "Grey" version, except we got sequels instead (which were amazing). To add on, every third version games are seen as optional purchases, the sales are reduced compared to the two versions. To remedy this, Game Freak wants less optional games in order to have more sales.
If a "Z" or whatever Gen 6 game were to happen, I just feel it would be scrapping the bottom of the barrel since Kalos has such a small roster of new Pokemon and the postgame was underwhelming to me. I really don't know what they'd add apart from Zygarde story and maybe Battle Frontier.
As for the Gen 6 mysteries, let's just hope SM will have them; I'm not too bothered about them to be honest.