Massive scale combat probably is too much to ask for given network and hardware limitations. However, what about zone defense? A map of the city would be shown as a grid. You win or lose grids depending on individual fire team results. The story can start by having a massive attack penetrating the wall in several locations. You can set the panicked atmosphere by having citizens run by screaming, etc. Random matchmaking or fire teams can pick city blocks to "hold the line" until reinforcements can leapfrog your position to push back the fallen. The timer would count down until the next group of guardians can make it to the front lines. If you get wiped out, Guardians lose the block and must win it back. The closer to the Traveller the Fallen get, the more concentrated they are requiring high level people. Further out, low levels can help because the fallen are more spread out. The more outer grids won, will make the inner grids easier to defend, making lower level guardian contributions valuable. It could be a weekly event resulting in the Fallen "winning" and gaining a foothold in the city or the Guardians winning and pushing out Fallen. New missions can be given out to help gather materials to repair the wall.