You could do WW1 MP, but you'll need to coordinate the players... why not make computer controlled commanders? They basicly stay at the trenches and gives commands like "Hold Position", "Take cover" most of the time, but at the point where enough soldiers gathered in the trenches they shout "Engage the enemy!", "Go" and so on.
If a soldier wants to attack on his own, he'll surely be mowed down by the enemys since the fire concentrates on him, so he will probably prefer waiting for the next "wave".
If people do well in an attacking wave they can gather more quickly for the next wave and advance faster.
This is just a rough idea, but making coordination forced by similiar to this could work out.