I doubt they would do that between gens tbh. But it'd be great if there was a patch for X and Y to make it compatible with new forms and items that Z/the next games introduce.
That's the point. Not just adding all the next generation's Pokemon to this game, but adding all the necessary files so that you could link the games together the way you could with R/B/Y and G/S/C, only without the "remove any G/S/C Pokemon from your party" bit.
Of course, the problem comes from the fact that there's an entirely new battle system, and while they probably won't change too much in the next game, there likely will be tweaks to the appearance. So instead of being able to copy/paste sprites like in the old games, they'd have to create entirely new models and animations in this game's style of the next game's Pokemon. That might make compatibility issues a bit difficult.
One thing it could do, however, is allow them to introduce new event Pokemon without having them already in the game's files. Right now, everybody knows what Pokemon are being "saved for later" as soon as they access the game data, but this way they could actually bring an entirely new Pokemon to the game.
I only meant updating X and Y to be compatible with other gen 6 games, not gen 7.