The topic of multi region Pokémon game has been beaten to death in the Pokémon thread: Two regions could work if they make the first Region more linear than Johto so that the level curve doesn't end up as fucked up as it does in GSC and HGSS, but after two properly paced regions you would be hitting the upper limit of the level curve (even more so with modern Exp. Share) and subsequent regions wouldn't work anymore.