Going forward, Dr. Strange is the next logical choice for a stand-alone hero. He's of a different flavor from the other heroes (magic, as opposed to miscellaneous science or mythological gods), so there's room for something new.
Lots of Ant-Man friends out there, but I don't know that he could work in a stand-alone. Also, we have to ask if he's really a different enough character to add to the ensemble. Troubled science guy? We got that covered twice. He'd have to be in a shared movie (like how Black Widow was introduced via Iron Man, or Hawkeye through Thor).
Black Panther is, I feel, a character whose time has passed -- the "everything African is tribal and jungley" era in comics. The only viable reason to run with him is the racial homogeneity of the current team. That leaves us Luke Cage... and I'm not sure he's the best way to go, either. A better choice, for my money, would be a recasting of either Dr. Strange or Ant-Man as non-white.
Dr. Strange could work very well under a different race or ethnicity, as long as we avoid the tendency to make "magical minorities" so overbearingly stereotypical (basically, don't make a black Dr. Strange look like the James Bond Baron Samedi). But then again, why focus only on black? Why not Hispanic Central or South American? Indian?
