Having the ability to actually pick a friend from the friends list in a different clan to attack. Because let's face it, game no fun along friends when there is no competing, challenge or threatening between them.
It could be done this way for example, each player gets to have only one attack he can pick on a friend a day. Also, if you don't want the other to lose resources or trophies you could add it just for fun. But including loots would be better though.

