With just one ROM isnt possible, because a ROM have limited space to have more trainers, pokemons, back sounds, etc...
The client will need more than just 1 rom, what is the best thing to do is creating 1 ROM per region.
But still that is alot of work and I understand why the devs didn't developed that yet, we have stuff misssing like the plant berries mechanism in hoenn, we need battle frontier, pokenav (And this feature can is identically to pokegear from Johtto) and other important stuff before going to the other regions I guess.
