Sound is a great idea. One of my favourite games for sound is the MGS (Metal Gear Solid) Series by Hideo Kijima. It's one of the top 4 or 5 game series out there.
In the game you can manipulate soldiers movements with sound, like rapping on a wall with your hand to get them to come off their patrol so that you can pass. You can throw your used up Socom magazines like a grenade, and the guard will follow it. With a silenced pistol you can shoot a wall to distract them, and without a silecncer there is a large radius that will give alarm. A guard cannot distinguish if you are an intruder if you are outside a cone LOS range, which increases with difficulty level. They will pick up binoculars and look at you if you alert them from a distance, during which you have a brief time to hide. You leave footprints on the ground when you stand in water. You must watch your shadow, and listen to their reports, etc etc etc.
There is very little stealth element in RTS games, it would be brilliant if you could say, hide your units under a cliff (and get rid of those damed outlines from AoK!). You could have units hidden under there that units above would not see. Similarly, you could have a terrain of long grass where you had low LOS, you could have the grass move out of place for a short time like the snow trails in AoK:TC, you could have higher LOS and range on a cliff/hill etc. It would add a lot to the game
I like your articles on realism Midgard Eagle, I read the one for EE on futuristic ships and such. Good work. Just remember, with some of your LOS ideas they would be draining on systems to implement if they are too complex. You need to be able to define them well. Like a crater being invisible would require a lot of vectors to be taken, and there would end up being LOS bugs when it got complex. Some like increased LOS when on a cliff etc would be great though, and easy to implement. Cavalry being a little less important as the game goes on, should all have a good LOS. Why can a scout see any further than an cav? And does holding a bow increase your eye sight?
AoE and AoK both had significant attack bonuses on hills. If you haven't noticed these I suggest you actually play the games. I noticed that melee units also have attack bonuses on hills very early on in zone games. How can any of you not have played Ulio? The lion and the cat? The joust? Come on people!