Stealing enemy base walls!!!
Since walls are so expensive, and you dont have to pay to rebuild them.... Why not make a special new goblin that is black, and when deployed he dismantles a wall and steals it so you can put it in your base! Of course it should not be easy to steal walls, and the higher level the wall, the longer time to dismantle/steal it. I think this is a great idea because for all the people with lvl 10 walls etc, they have nothing better to spend all their money on but building new walls. ;)