Well, it's a good idea overall, but it has two major set-backs that make it impossible:
Profit: Nintendo wouldn't ever let this happen. It's similar to how the 1 Game = 5 Regions would never happen. They would never be able to capitalize on remakes and stuff alike. I mean, remember RSE? You would never be able to use pokémon older than Gen 3 there, you wouldn't have the physical/special split, some moves mechanics are different, the game mechanics are different in general, and it would make remakes seem pointless, except for graphic and maybe plot changes. It would also make "catching 'em all" and trading for evos (with yourself) waaay easier. I caught all pokémon you are able to in every game I had (no trades) in only one, so I would've caught em all by know, if I had friends with other games to trade with/get other starters from and it would also take away that part of the games.
Game: Like I said before, you wouldn't be able to use newer pokémons, even 4Gen ones, in the old games. Reasons for that are pokédex limit/size, like said, evolutions would need to apply there (you can now trade Feebas with specific item to get Milotic, but before you only could max out its Beauty status). Also, what about newer items (be it pokéballs, key items)? What about stuff you got in the older games, how would you be able to transfer it to the newer ones? What about sprite editing so you could see the newer pokémon with 3Gen and 4Gen battle screen? And the HGSS overworld sprites? And your sprite? It's waay too hard to do, and that's what they have remakes for, like I said.