Foxx is a good choice, as good if not better than Elba. I haven't been impressed with anything Elba has done apart from The Wire. Will Smith has typecasted himself into PG-13, non-threatening Negro roles and would therefore not be an ideal choice. My ideal choice is Don Cheadle, but keep in mind that the market and potential box office take are frequently behind casting decisions. Cheadle is a superior talent than Foxx, Elba, or Smith, but not a money maker on opening weekend.
The biggest star in terms of box office potential is Leonardo Caprio, an L.A.-bred pretty boy with a babyface who dates foreign supermodels, rolls a Prius, and regularly advocates for progressive causes. Unless there is heavy makeup involved, I don't envision him as the best choice to play a brutal slave master in the antebellum South.
All that said, I expect the film to an excellent pseudo-historical revenge fantasy along the lines of Inglorious Basterds, because no one does dialogue like QT.