"Pok?mon Z" isn't official; it's speculation and rumour. As of now, Game Freak hasn't announced any plans to extend the story or features of the sixth generation games, and with that being said, it's not guaranteed there will be a Z version.
There were rumours of Pok?mon Gray, but those were proven false with the release of Black 2 and White 2. So it's hard to say whether Game Freak will extend upon X and Y as they have done in previous generations (such as Yellow, Platinum and Emerald) or opt in favour of a remake to a previous generation.
Only time will show what Game Freak decide to do; agreeably there's still potential for what can be done, but X and Y are still recent (having been released in October 2013). Based on that, a third game is usually released about a year later... so we'll see what happens. Logically, it would make sense for a "Z" version.
A lot of rumour and speculation that's floating around include "Pok?mon Z", a Pok?mon game for the Wii U, seventh generation "Plus and Minus" versions, or generation 3 remakes. What comes next is anyone's guess.