If the game takes place several years after the events of BW, it would be cool if natural disasters struck some of the places which greatly altered paths or possibly opened up new caves.
It would be really cool if the changes were version specific, taking Opelucid's concept further. Cities will now be more technological in B2, while more natural chqnges happen in W2. In W2, new areas would be uncovered by an earthquake (or something), while in B2, those same areas would be uncovered by construction.
Also, their should have been new HMs in BW1.