The Arena: attack the bases of your friends
One of the things that makes gaming incredibly fun is to be able to interact and play with your friends. What I feel is missing is that I can't attack the villages of my Facebook friends/clanmates. Adding this option to the game would increase competitiveness among players, make the game more fun in general, and enhance the gameplay of many. Just think of games like mario kart or super smash bros: playing against friends is what made these games so much more epic.
Idea of how to implement this:
Add an 'Arena' button. In this screen, you see a list of your Facebook friends and clanmates and you can challenge their war base. If they accept the challenge, then you both have 24h to attack each others war base. The one who wins most stars wins 150% of the costs of his attack. The loser gets 50%. So the 'award' is really insignificant, it's not possible to make money out of it, it's just about having fun with your friends and about a battle of prestige.
Sidenote: the players are not able to change their war base. the war base is the war base that they have used in their last war. This way, it's not possible to copy a base of someone you're currently in war with and practice on it.