BW2 cartridge was like the action replay's and you put your black version in blk2 and whte version in whte2 and u continue from where you left off in unova but theres a new story and extra islands to travel to, like the sevii islands were to firered
And maybe more land access up north of unova and has a battle frontier
Although that does seem like an interesting idea, it would not only cause sales to be horrible, basicly requiring people to purchase black or white beforehand, but if you went for 100% completion storyline-wise, you probably have a team of pokemon that is somewhere around average level 75ish. this would mean all opposing pokemon in b2w2 would likeley be at that level or higher, and tbh, that would blow.
Plus, i agree with IceDragon, i dont want a giant thing sticking out of my 3ds. im sure they could develop a way for you to import your b/w save file onto the dsi/3ds memory card, and then import it again to the b2w2 cartridge. but again, importing an intire save file would cause for a horrible game.