Okay, so I'm having trouble seeing Djimon Hounsou playing the Doctor. (He seems a little too intense.) Chiwetel Ejiofor (from Kinky Boots) would be my personal choice.

And Sophie Okonedo is awesome. I loved her in Dirty Pretty Things, which starred Chiwetel Ejiofor.

Funny, when they started talking about Torchwood USA, my first thought was casting Benjamin Bratt as Captain Jack because I thought the show could use a little color.