This is similar to the map editor mod, in that it might be better if implemented by the developers. That being said, it would be pointless to have objectives unless some of these other things were implemented. If you don't have unique units and you are not given greater control over both individual units and the larger horde at your disposal, then there wouldn't really be much player input.
However, if we do have all this other stuff at our fingertips, then objectives for custom levels and maps could add a whole new wrinkle to what is possible in the game. For instance, maybe each unit type is worth a certain number of points and you are tasked with composing an army that is worth X number of points that can conquer a preset enemy army. This might require some actual game balancing, but if a niche community built around the idea, then it could become a great time.