Well, Terry will need to be replaced by 2014-15, I think. I think he's still a good defender, but he's no longer 1st choice. Our first choice partnership at the back is Sideshow Bran. Cahill and Terry as back-ups is more than enough, especially if we have a capable, young defender to act as 5th choice when Terry inevitably gets injured for trying to head someone's knee. Next year Ake should be loaned to Vitesse or something, but we could use Bruma or Omerou. They are both promising and can play multiple positions across the backline and would fit perfectly because of that. That said, while Omerou has been great at ADO and established himself as an NT regular this season, Bruma has failed to live up to expectations and hasn't managed to hold down a starting berth at HSV for two seasons even though their defense is terrible (Mancienne for crying out loud). He might still become a good CB, defenders peak later than attackers, but his contract runs down next season so loaning him out is pointless. Either we sell or we keep him as 5th choice for a year and if he proves himself we extend him. I'd say loan Omerou because he shouldn't be 5th choice, he could be starting for a promoted club next season, I think. We also have Ake, Kalas and Christiansen who are all excellent prospects (although at different points of their development and it isn't clear than any of them will actually be a CB when they mature).
So, with that in mind, what I'd do is loan out all the CB prospects to better clubs (Kalas and Omerou to Bundesliga or Pre, Ake to Eredivisie, Christiansen I dunno, maybe another year at the club) and keep Bruma. Play out next season with this squad, let Bruma leave if he doesn't prove himself and extend Terry for another year if he's still capable of a bit-role. Then promote Omerou/Kalas or purchase someone and possibly sell Cahill a year or so after that to make room for someone with more potential (although he's perfectly capable).
Right now, I don't think there are many partnerships out there who are better than the Sideshow Bran (Dortmund, Bayern, Barca, Madrid, City, Juventus are the only ones who might, and I don't think it's 100% on any of them), but, since Ivanovic is also one of our RBs and DL is one of our DMs, we would do well to get someone else. Were we to buy anyone right now, I'd say that anyone at a Top 4 contender, Bayern, Barca, Madrid is out of our reach. Or at least too expensive to be worth it. There are cheaper, younger options are the world and we could always try and lure Subotic/Hummels away from Dortmund. Pepe, assuming Madrid want to be rid of him, would be a great choice, talent-wise. Personality-wise, bleh, we have David Luiz to level him.
Also, Chelsea have the team thread with the most posts in 2012-13, winners!