The idea would be to have another immortal, but they're based on/level up through experience in battle rather than dark elixir.
For example, every building they destroy/troop killed is 5xp and they need a certain amount to get to the next level. My only concern would be their 'cool down' or 'building time' between levels. Obviously people just gem things now, but slowing them down with a build time could make it more interesting.

I'm not sure if this has been suggested before or not.