A lot of people mentioning Mangala. But from what I've seen of him in the Europa League run (he was horrific at Sevilla, average defensively at home to Napoli (better away!)) and what I've seen from Demichelis in the last few months, I'd MUCH rather have Demichelis.
Mangala looks unsure of himself positionally, he is only young but I'd rather see Nastasić bedded in to our tactics than spunk a load of money on someone who doesn't look fantastic (for £35m+!) but rather stick with Demichelis until someone really special comes along.