If you ask me, I'm not much of a story writer myself, and I think designing levels which suit linear gameplay takes much more effort and thought than creating locations for emergent battles.
If this was to be added in the vanilla RWR, I would seriously need a helping hand for writing and doing level design. At this moment, I really can't afford that and besides, the game development focus is in another direction.
I could see myself adding support for mods to achieve this kind of thing, if I knew exactly what would be required to define simple missions.
You'd obviously want some control over the bots, what they do, where they are, what's the respawn logic, and so forth. Also, you'd need a way to define some triggers for certain events like mission success / failure, perhaps sub-objective completion within missions, and the triggers could be to kill/destroy specific target(s), get specific targets to a location, and the targets being soldiers, vehicles, pickable objects, etc.. As you can see, the possibilities grow pretty easily, so to keep it simple and get it usable as early as possible (not saying it would happen soon though, maybe ever) I would need to know exactly what kind of missions need to be possible and how their components would be defined in level design perspective.
I could guess that the missions would be mostly based on just the player(s) + their teams, no fellow AI teams, so it would be more like you vs the game, right? This way you could skip defining what your "faction" is doing while you're on the mission, but I can easily see mission types where you'd want to see fellow bots doing something other than just your squad.
Thoughts?

Again, I just want to stress that this won't happen in a while, as the game has been made as a non-linear simulation type from the start and this game mode is not finished yet in any way. Making a linear story driven game mode that has very little to do with the current logic wouldn't make any sense at this point. Saying that, I'm in no way against such a mode.