You know, the best thing about this generation's version names is the double meaning it can have.
X and Y are -- along with Z -- the axis coordinates that can be found upon a map/3D plane.
X and Y are also the chromosomes present within mammals and such in genetics.
Even if there is POSSIBLY a third version that ends up titled 'Z', it still wouldn't break the chain. However, after Black/White, it's unlikely they'll do a third version again, and would probably do a pair of sequels or something. In which case, the sequels could be Z and W
Z and W are both part of coordinates to some degree. You have Z which is part of the 3D plane, and then you have W, which can denote dimensions in regards to homogeneous coordinates and such.
Z and W are also the chromosomes present within birds in genetics.
Gen VI has the best titles.