What about a scripted/sided multiplayer match? It feels like a single player mission, but the enemy mechs are controlled by enemy players. Obviously, one or two players play the single player side with 65-100 tons mechs, but everyone else controlls the enemy mechs/vehicles. When one dies, they wait to respawn with the next triggered wave. When they are waiting, they can control cannon fodder tanks and the like. Whoever does the most damage gets to play the single player mission on the other side.
This whole idea makes it possible for ~5 players to play a big engagement. The 3 "enemy" players end up playing multiple small mechs and vehicles, but the single players with their single life/vehicle think they are facing the horde. Obviously, the enemy mechs would be much smaller, or if an enemy mech was close it size that player would not respawn in another mech.