I don't play as many RTS games as I used to. I've got Age of Empires: III and love it, and was impressed by Battle for Middle Earth II, but my interest in RTS has waned after C&C: Red Alert...and I'm honestly not sure why. So maybe the following thought is out of ignorance, but it applies to the games I know.
RTS controls are in dire need of innovation.
Positioning an army exactly how you want them in these games is like trying to parallel park with two flat tires. First I have to ensure my army is coming at the target spot from precisely the right angle, and then I have to be sure to click on exactly the right spot...not too far ahead or behind. A failure to position the army right where I want it means a gap in the valley or other zone I was attempting to block, or some other strategic fault.
Why can't I use the WASD keys to maneuver my army precisely, as if it was an FPS character controlled in 3rd-person view? Why can't I use the Numpad keys to quickly and easily instruct my armies to maintain a particular formation including mid-march, and not just when they reach the target formation? Wouldn't it be great to change their formation in reaction to a moment's circumstance with just the tap of a single key, instead of having to rush through a series of scroll options (like in BfME 2)? I seem to remember Sacrifice doing this to some success.
And wouldn't it be nice to have the option of preset strategic commands of a script beyond a single action?
Imagine having one hotkey for "fall back to the valley" and another for "fall back to the fort"...these points being selected by the player through waypoints.
Imagine being able to direct each type of military unit (archers, pikemen, cavalry, etc) to focus its attacks on a particular unit type of the enemy's (ex: "my archers, aim for their cavalry first"). This could be selected before battles and would remain active until otherwise chosen, like character stances in some RPGs and fighting games. There could also be items or hotkeys to allow players to affect unit-enemy preferences on-the-fly.
This could also be applied in the other direction. The player might direct particular units to retreat only if they encounter a particular enemy unit.
Imagine if the player could issue orders that remain latent until conditions, chosen by the player, activate that command. Ex: "If > 40% casualties, fall back to the valley." That command could even be compounded with "If > 70% casualties, fall back to the fort"; compounded meaning that both commands were issued before the battle, but are merely activated separately (and automatically, in response to those conditions).
It would be great if I could rank my units as hierarchal lines of defense. As games are now, in order to have my pikemen stay and guard the retreat of my archers, I must have the two types of units separately selected and accomplish the retreat manually. But what if I had preselected my pikemen as the first line of defense and the archers as the second line? So when I order the retreat, the pikemen automatically know to stay and fight until the archers are a safe distance away.
Controls like this, especially, would make RTS games ultimately more easy to manage. Of course, all of these simplifying controls require a learning curve, so it's debatable how much this lowers the initial learning curve. But certainly, more casual gamers could be allowed a fair gameplay experience without use of many of these features, while veterans and hardcore gamers could explore an extensive range of strategic possibilities and command prowess.
One of my favorite movies is Patton. I highly recommend that and other old WWII films to anyone interested in RTS design, because they help expose the many limitations of current RTS games and the avenues of potential growth.