Nintendo employee here.
Work is already underway on BlazeRed and SproutGreen(latter is tentative title but may turn permanent, last I heard it was between this and ShrubGreen). These are going to be the last games of the gen. Work is also underway on sequels to X and Y for Gen 7. GameFreak's two teams are working on these consecutively.
Starting from XY, we've been working alongside the Japanese developers to add the English language script as they make the game, so that there can be a worldwide release. From time to time we get sent builds of the game with the areas that they want translated. However we do not have full insider information as to what features they are planning, etc. The game is further along in development than what we receive, we get the areas that are for the most part done so we can make the script. We also work on other projects aside from Pokemon.
The early build we have at the moment reuses battle backgrounds from ORAS and goes up to Pewter City, with Viridian Forest unfinished. There are no major engine changes, e.g.. the framerate drops are still there. One change is that the opposite gender character currently meets you on Route 1. The Male Character is almost exactly the same as in FireRed/LeafGreen, while the female looks more like her manga counterpart, with a black dress and . They are both somewhat unfriendly when you first meet them. There is also a man telling you to pick up Oak's Parcel upon entering Viridian City.
The rest is essentially the same, the League can't be accessed however you can see a bunch of giant statues over in the area.
