No, Giancarlo Esposito
wasn’t cast in nuBSG, but I could imagine him playing Cylon Brother Cavil had Dean Stockwell not been available for the role.
And it would have been interesting to have seen EJO’s Bill Adama and Esposito fighting it out since an Esposito-played cavil would likely have been more fight scene oriented.
Of course beside Cavil, Esposito would have made a good Ha'la'tha mafioso in "Caprica" alongside Esai Morales's Joseph Adama, or even the role of a black market ring leader in S2's "Black Market".
I got this impression after seeing Tom Neville in “Revolution”, and of course Esposito portrayed Gus Fring in “Breaking Bad”.
This is not to say that I didn't like how Stockwell portrayed Cavil.
In fact I like Dean Stockwell's portrayal of Cavil quite well and I'm quite satisfied with Stockwell's character. And of course Stockwell's Cavil has some of the most memorable lines of dialog in BSG too.