It must be playable for casual and single player gamers. I’ve never seen a casual friendly multiplayer RTS I believe. The FPS genre manage to do this, so I think it should be possible.

Nothguard, Dune: Spice Wars, Line Wars and maybe Total War, could be considered casual friendly multiplayer RTS.

Though I must admit I want a good e-sport RTS I can watch and maybe dip my toes in, now when SC2 and AOE both feel quite dead. It’s the only e-sport that really entertains me. (we’ll maybe chess is an e-sport as well nowadays)