Never.
First of all, the Japanese logos have a little DNA symbol in them, so I am taking that to mean that the games are based on X and Y chromosomes, not the three-dimensional planes, XYZ.
Second, the B2/W2 games were very successful, and (in my opinion) were far better than a Pokemon Gray version would have been. A new story, expanding the region significantly, etc... It was a much better idea than just an updated rehash of the same game.
If anything, we should be expecting Pokemon XX and XY, or Pokemon X2 and Y2