Player-controlled turrets. That would be gnifty. You cover the builder long enough to set it up, then proceed with the ownage.
Or, on the same note, laser-guided rockets. Small-wield rounds, wich are launched by playerA. PlayerB then uses a laser guidence system to target something.
Hmm, another idea: How aobut player-controlled rockets? Player launches the missile, then controls is using a camera on the missile. This way you can stear the things around corners.
Ah, another idea. How about the ability to choose different payloads for missiles? There would be multiple missile types: Incindiary, nerve-gas, shrapnel, guided incindiary, guided nervegas, guided shrapnel ect.
And the ability to upgrade turrets would be cool... Upgrade a machine-gun turret with better rof, or tighter/wider spread. Then, you can upgrade it again. It would be an upgrade tree. From RoF upgrade, you can go to more damage, or tracers. From tracers you can go to better tracking, or something else. That would ge cool.
Oh, and another question:
How are structures going to be made? Will you place the structure, then "repair" it untill it is complete, or just place it as a whole, and it be done instantly?