The more i think about it the less i think it to be a "cheap" idea but what about the ability to "upgrade" structures and furniture?
Example being wood walls => metal, wooden doors => powered, lamps => colored lamps
Rather than having to micro manage like crazy, make it so you can disassemble and then build in its place without the leg work in between.
Now for some real cheap ideas:
Shrapnel Grenades (Weaker than frag but only does small amount of damage to structures)
Revolver (Same range as pistol but slighter slower and higher damage)
"Party Crasher" Event (While getting attacked/while they are still preparing to attack, another hostile faction drops in at first factions current location)
"Warning Signs" (Hostiles take a hit to their moral when passing large amounts of graves)
"Dinner and a Show" Event (Two opposing factions appear on the map and then begin waging war against each other)
Dump Storage Energy (Lets you dump out some of the power in your batteries to prevent power surges)
Alt turrets:
Artillery (Not sure how this one would balance out but would be similar to mortar but unmanned and weaker or very expensive)
High Caliber (Fires a single, high damage shot)
Heated Shot (Like the Basic turret but shots have chance of starting fire)
I've been playing this like crazy so I'm sure to come up with more ideas ^^