Mercenary Barracks and new troop
You how there is elixir troop and dark elixir troops, there should be a Mercenary set of troop. you have to pay a large amount of gold for a few overpowered troops that can survive one or two battles
the normal deployed troops die at the end of each battle, but the Mercenary survive the battle with the amount of health they ended with.
a second idea is a new troop, could be a Mercenary maybe. the troop is a dwarf miner. he acts like a normal barb but he breaks though walls faster then normal troops without killing himself. he does extra damage to gold storage and defense turrets
he fights with a pic and when you max him out, he hold a warhammer that can deal damage to walls aoe