I'm happy to see the number of people who've already stated the possibility of a Z-version sequel. Think: why was it necessary for Black/White to have two games as a sequel instead of one?
1. The plot diverged at two different points in Black/White, with N taking a different legendary pokemon which directly affected Ghetsis's plans in Black/White 2. Meanwhile, in XY, the plot ended at pretty much the same place.
2. Black and White had varying landscapes--technology or nature--that carried over in their sequels.
Although even then, there could have hypothetically been a Gray version which combined the 2 as a middle-ground for the two to coexist...
I'm going to vote in the poll in favor of a Z version, simply because it's the less-favorable option if it doesn't also become a sequel...