Pokemon BlazeBlack and VoltWhite 2 are really well done mods. The modder managed to balance a lot of the pokemon and their moves and make all of them available in the game (plus all of the legendaries in a way that makes sense, complete with overworld sprites), increase the difficulty of the AI, and add several new trainer battles as well.
The regular Black and White 2 feels really lacking after you've been playing this mod for a while.