Here's how I think Black 2 and White 2 are going to work:
~The game will primarily take place in Unova.
~The playable characters will be completely different people to be introduced; they may even introduce customizable characters for the first time but don't count on it.
~The story will be a continuation of Black/White's storyline, not a retelling.
~There will be a new subregion similar to Sevii, possibly south of Unova. You will start in one of its towns, but you will not be able to explore the subregion in its entirety until later in the game.
~After recieving your starter and Pokemon and Pokedex, you will take a ship to Castelia City and pursue the Unova gyms, but in a different order than in Black/White.
~The starter Pokemon will probably be the same ones as in Black/White. Maybe they'll get new forms to provide some originality--doubtful. Or maybe we'll get to choose from the monkeys; I highly doubt it, but this is what I would prefer, because I have always disliked the fact that the starters we get from the beginning of the game are some of the strongest Pokemon in the game. Were we were to get a weak Pokemon from the beginning like the monkeys, we would actually team-build throughout the game instead of just duke out our starter on everyone.
~The playable characters from Black/White will be NPCs involved with the story. The BW character who corresponds with your gender is likely to be the "ultimate" battle (similar to Red in GSC/HgSs) and will have Reshiram or Zekrom depending on your version (Reshiram if Black 2, Zekrom if White 2).
~The Kyurem you will catch in this game will be a fusion of a newly awakened Kyurem and N's dragon from your game's prequel. N will not have control of it and you will be able to capture it.
~There will be a fourth form of Kyurem that will essentially be the Tao trio combined (such a Pokemon is hinted-at in Black/White). It will require you to have the dragon of your game's prequel in your posession and it will be badass. Or maybe it will require you to have the Kyurem form opposite to your game's mascot. Either way, it will be done by GameFreak to give you incentive to buy the prequel for or the opposite version of your game if you haven't already.
Making sequels to Black/White instead of a third version is a brilliant franchise-milking technique. Scripting a new story will take a little more work than the normal third iteration, but they'll get over twice the amount of sales, while still being able to recycle the entire region and all the sprites, 3D models, music, and of course game engine. Plus, the fans will be much more pleased with a sequel than the would a third version. Brilliant move; I hope this is a pattern to be maintained in future generations of the franchise.