One of the things that has always interested me in RTS games is a unit ranking system. I first noticed in Starcraft, where units didn't gain ranks, but they did have the "Private", "Corporal", ect. which, for me atleast, was awesome. Other notable examples are games like Command and Conquer; units gained ranks from how many enemies they killed, and it would give them increased damage/health/other bonuses ect. I think a system like that would be awesome in 0 A.D.! Also, this is a bit of a longer stretch, but I've played RTS games that have allowed me to change/swap equipment for my units. Not like Men of War, Soldiers of Anarchy, ect. where you control your units inventory, but more of like allowing a unit to swap between a bow and dagger/shortsword, or something along those lines.