The problem with that is, the level balance for all the regions would be incredibly wonky.Another unlikely but cool possibility would be a game using each region. You start in either Kanto, a new region, or your region of choice, and then go through the gym challenges like normal and then start going through all the other regions in the same manner. Never gonna happen, but hey, it would be sweet.
Think back to G/S and Hg/Ss. At the end of the Johto campaign, the highest level trainer was Lance in his Lv. 50s. At the end of Kanto, before Red, Green/Blue's level was in the higher 50s.
Now, if they did create a game with all the regions, that means the level cap for each region would have to be reduced in turn, so by the 3rd region, the gyms need to be at around 60-70, and etc.
... It just seems rather odd to do that, since one of the regions I disliked Johto was the fact that the game goes a tad short from the others, unless you counted Kanto with it.