I think it's safe to say that most of us think that Pokemon Sun and Moon are a part of Gen 7. With that said, I still don't think that a 3rd installment in the main series of Gen 6 is out of the question. There's clearly still interest in Z version, Nintendo might want to capitalize on that. We have a new anime on our hands, Pokemon XYZ. Some might say that that might be there just in spite of a lack of a 3rd version to begin with, but then we have all these new forms of Zygarde, the Legendary Pokemon that would be on the front cover of a potential Z version. It'd be pretty weird to introduce these forms in the next Gen, when they clearly point to Pokemon Z being a thing.
I'm not sure when this game would potentially release, maybe Nintendo will announce it at E3 along with the rest of Sun and Moon. Pokemon Z could launch in the Aug-Sep range with S&M (yeah, I guess that's our new abbreviaton, get used to it) launching in November. It's hard to see a Z version releasing after S&M.
What do you think? Would you buy a Pokemon Z, even with Pokemon Sun and Moon on the way?
EDIT: To make this clear, I'm not a huge fan of Pokemon like some of you are, and I wouldn't buy a potential Z version.
bet: lost