I would take a cue from coup de grace, since anyone can do that under the right circumstances, and offer an ability where if a fighter has an opponent pinned downed or restrained the hurt train starts rolling.Hmm, what if they gave the Fighter the ability to really summon up a lot of fighting spirit and attack everything in front of him for a set amount of damage, say 3d6, with half that much on a miss. Start him out with 2 uses of the ability per long rest, but give him more uses and crank up the damage as he levels up.
We can call it: Burning Hands.