Yea, I thought you were a bit thin, particularly in the centre back department. I can only really think of Milito as a back up CB unless you move Abidal inside. Caceres is still a Barca player as well isn't he? How's he been playing this season?
It was more just the idea of improving the first XI that crept into my mind. Pretty much every team can always sign a player who will improve their first XI, but I couldn't really think of one for Barca, with the exception of maybe Evra or Casillas, with the likelihood of the latter happening being about 0%.
EDIT: Aren't they trying to groom Thiago to be Xavi's replacement?