There possibly will be a game like that. However, the only problem that I will have with the game is that it won't really be challenging in some areas.
What I would like is that instead of predetermined levels on their opponent's Pokemon level, they would take the average of our Pokemon and make it the gym's Pokemon level.
We started in the Kanto region, defeated the Johto region part of the game and am finally moving on to the Hoenn part of the game. Our pokemon should roughly be around level 60-63-ish.
Now what if we want to go back to Kanto and get 100% completion on the trainers? Their pokemon would be like... level 10-12-ish? So if they implemented that function, their pokemon would be the average of ours, so it'd be like level 60 as well. It would be fun and more... challenging. Think of the Guild Wars 2 area progression in a way.
I kind of went astray and suggested an idea that I hoped they would implement if they do create a game that is based on all regions.