I think someone more aggressive and dominant would bring out the best in Silva and give Milan's backline the best balance.
Ideally, that player would also be a leader/organiser.
Stam.
Havent you seen enough of Silva this season to convince you that he is on his way to becoming that?
For me Zapata is the ideal partner for Silva. Silva, as we have seen, is a thinker. He anticipates his opponents moves five steps ahead, hence why he is in the right position at the right moment almost all the time. Yes, Silva can also be the marauding CB ala Lucio if he wants to but it would be better for Milan if he is the more sensible type and is a leader at the back (especially if he is to be our future Il Capitano
). Zapata on the other hand, we can afford him to be more of the impusive type. And I dont mean the crazy stupid Mexes type impulsive, but more the more adventurous type of impulsive who occasionally helps the attack while Silva holds the fort at the back (cauz let's face it, he is the ONLY one we can trust to do that). That doesnt mean that Zapata would be solely the attacking type CB, he would still need to do his defensive duties (and that too admirably), but it also means he will not be completely restricted to it.
I feel Silva can take over Nesta's role imo. He is the best positioned and suited to do so (considering he is the apprentice of both Nesta and Maldini) and also as he would be the senior-most member of our defense after Abbiati. Plus if some may have noticed, he is already on his way to become what Nesta was/is for Milan albeit with a different approach.
Zapata on the other hand can be the new Silva. That isnt meant to say that Silva should change how he approaches his game, definitely not. But he is also better equipped to handle pressure situations which makes him more suited to take the backseat in the partnership. Zapata would be the new Silva in the sense that he would be the rookie of the duo, and would be less burdened with defensive responsibilites and arranging the defense...
So I dont see why we need a new Nesta to partner Silva when we already have him