Pirate, probably a good read
To go with the theme (BK for Barbarians, AQ for Archers), I envision a Goblin decked out in pirate gear. Because of his shady nature, he's never around when your town becomes under attack...but if his base is destroyed during the attack, he will not be available as if he took full damage.
The "guard" button is replaced with a "mission" button, which will bring up a troop donation screen that you can choose what to send with him on the mission. The pirate and the troops leave your town (similar to when you donate to your clan), and after X hours, only the pirate returns with gold (stored in his base for collection) and is unavailable as if he took full damage. Each upgrade will increase the troop capacity, time it takes to return, and pirate recovery time. The amount of gold returned is 80% of the elixir cost of the troops sent (the % would need balancing), but allows for a one-way only transfer of elixir (which is overly abundant in the game) to gold (which is constantly depleted).
If you use the Pirate in attacks, rather than missions, he is rather slow (like the King) but attacks much faster with a rapier at less dps. When attacking troops, he can hit multiple enemies at the same time with the finesse of his blade, but when attacking buildings, not so much. His favorite buildings are laced in gold (mines and banks) and will do double damage to these buildings. At level 5, he gains the "Golden Chalice" ability, which heals him slightly, goblins emerge from his location, and enrages all goblins in his area of effect. Every 5 levels increases the number of goblins and other effects similar to the BK and AQ.
I have other unit ideas, but I thought I'd place this one separate because...well...it's awesome and you know it
:)