I quite like the tower defence training level in brink so what if Splash Damage were to make a never ending one so you have survive wave after wave of enemies before they take the command post in the middle. It can be done with 4 people so one per class. The Soldiers resupply people, the Medic keeps everyone alive and the engineers lays all the defensive stuff and buff people's weapons.

That leaves the operative with something to do and I figure allow the operative to hack doors in order to close them for a certain amount of time so enemies cant come through them, and also firewall the command post if they can.

Also maybe the amount of command post to protect can increase and the team has to have every command post under their control or else they lose.