The Lucifer Cannon is to us what the BFG was for Quake or Doom. It's a heavy weapon that can be charged to fire a slow but devasting energy projectile that will blast most aliens to bits and can tear down whole bases.
Have a look at the concept by alex_bo:
The weapon has a single battery and does not need a reload animation (it will be recharged remotely once you get into the range of an armoury).
Note that if you're indifferent about what human weapon to model, the HMG would have a greater priority for us.