I have no idea.
I'll probably trade some of my peeps that I don't use often, from Black1.
I'm not sure if Black2 introduces new Pokemon, except for that crazy Cassette Bug pokemon (Genesect) and that RockStar-looking one (Melodica?), but it DOES seem to bring back locations for the Pokemon from older generations that were not found naturally in Black/White1 (like the ones I'm missing; the Sinnoh Legendaries, etc.).
If you recall, after you're done with Black/White1, the Team Plasma ninjas give you the orbs for Dialga, Palkia, and Giratina, but you never meet those dudes. In Black 2, you do.
Also, I like that you will be able to have battles with the gym leaders of every existing region (BROOOOCK!) via some kind of online system in-game.
PS: 4942 6024 3661 is my PKMN Black FriendCode, in case anyone would like to trade or have me raise Pokeys.
I don't really battle much, though.
PPS: 3095 7196 3065 is my PKMN White FriendCode.