newsflash (not really, if you want real news, don't read this):
they're not going to make a game that allows you to travel between all the continents, they MAY revive GSC, but they WON'T be making all 4 areas availuable in one game
the johto games allowed you to return to kanto because they had trouble with backwards compatability, the DS and GBA are perfectli compatible, infact, P/D requires you to use the old GBA carts for a complete dex
i don't mind them recreating GSC, nor do i mind them removing the kanto region (idea: if fire red/leaf green is in the GBA slot of the DS, you unlock kanto region) it's been so long since i played GSC, i already forgot the whole thing XD