I don't have a fully-formed concept or outline for a game yet, but I do know one thing I'd really like to see in an RTS game.
I want to see a black African civilization as a playable faction. While black ciivilizations have appeared in some turn-based strategy games (e.g. Civilization), they're relatively rare in real-time strategy. In fact the only two RTSes I recall having black factions are Rise of Nations and the Empire Earth II expansion pack, both of which are years old now. If RTSes had a greater ethnic diversity of factions, then the fanbase will increase in breadth and diversity as well.
Some African civilizations that could be used more often in the RTS genre are Nubia, Ethiopia, Mali, Zimbabwe, and maybe some of the Swahili kingdoms. For that matter, even the more "tribal" African cultures like the Zulu or Maasai could make for interesting if unorthodox gameplay.
However, perhaps the black civilization I'd love to see in an RTS game the most is Egypt. A lot of people don't realize that the ancient Egyptians were mostly dark-skinned Africans (albeit not quite as dark as the Nubians to their south) who would qualify as "black" by American social standards. Nothing would make me happier than an ethnically accurate portrayal of ancient Egypt in an RTS game.