It sure would be cool, but then the expectations for every new Edition/Gen would be so high that GameFreak would be forced to do that with every future Pokémon game and end up always having memory issues which porevent them to dedicate more time into the new region.
"Evil, beware. We have waffles..." - Raven, Teen Titans
Don't get me wrong, I'd love a game like this. I just think that the drawbacks outweigh the good points. I wouldn't want them to do it until it would be a brilliant game, rather than a mediocre attempt. Otherwise all the people currently asking for a game with all regions will start moaning about how rubbish the game was.
Ash Ketchum is a Timelord; he never ages! That or a vampire... Official Umbreon of the Eevee Corps *Evil Laugh*
You ever heard of Pokemon 3D? It's far from complete, but there are mods also in development for pretty much every region. When they are eventually completed it might be something similar to what you're looking for.