I think that you should have a terrain-leveling system to make hills and the like and trees aswell. Also, you should be able to change the theme ie woodland/desert/ now etc. Maybe some environmental effects like snow /rain/fog etc.
Also, switches, trigger boundaries and maybe a generator that when activated/destroyed/stepped into it changes something ie a previously locked door becomes unlocked/a shield door dissapears/auto-turrets come online.
Maybe some form of scripting tool so players can make their own campaign/firefight/challenge maps.
These are my ideas.
