I think the casting would depend on what kind of character the captain would be. It seems like there would be two main types, the younger maverick type, ala Kirk, and the older more experienced type, ala Picard.
If I were in charge of casting here's who I would like to see audition. For the Younger route: John Simm, Joe Flannigan, Michelle Rodriguez, and Scott Cann.
If I were looking for someone a little older with more gravitas: Sela Ward, Christopher Eccleston, Dennis Haysbert, Mandy Patinkin, and Tim Dekay