The east half of Unova will get a purpose and become an important part of the region for the main story,
instead of being a long path with nice but boring places filled with random high level old filler pokes.
Heck even Kyurems original spot is in that mess with clefairies and wild beldums .____.
BW2 gonna fix that and make the WHOLE game feel properly fleshed out instead of just up to E4.
The starters will stay the same.
Gyms will definitely have some freshing up (or even a leader/type or 2 changing).
Obviously we won't play the same hero (we WILL have to meet the old one with his dragon at some point).
New locations are a given too. Subway needs more use, as does the airport.
The only really big question that has to be answered before anyone can take it serious is whether or not there will be ANY actual new pokemon.
There is no way there would be a proper generation with hundreds of creatures for this however.
The only possible twist would be a radical "mid-generation" with say 50 or less pokes to complement Unova, while Unovas 150 still being the common thing. It would make things a lot more interesting and justify the "2", but again, not to be taken serious unless it turned out to be the case..
It should be like Gold/Silver but better thought through and planned from BW on.