A couple of years ago I was at a C&C3 Community Summit. The developers took us to dinner and at some point the subject of mods came up...with the dev's agreeing that the first game to be the Quake 3 of RTS would be huge.

It looks like that game is going to be Starcraft 2.

The following screens are from Starcraft 2, not Starcraft: Ghost...not some Flash game. These are unpolished mods, made by the SC2 devs with their worldbuilder to show what is possible. Click the image to see the youtube clip showing these off (starts at about half-way in).

Even with the fact that they have set it up to allow for 3rd person shooters and RPG's, the biggest takeaway I had was the flexibility of UI modding. The fact that you can add in new UI elements (like the boss life meter) will add a variety of options such as tossing in C&C styled superweapons, ingame reputation meters (ala WoW), and a ton more.

In update to the last SC2 post I made, yea we'll be able to add in custom art...but it won't likely be available at launch. I can wait though, since it'll be a very good time to be an RTS modder.