Friendly battle & local clan tournaments
The December update damaged the social aspect of CoC. Many clans talk about this on the forums. Here is a feature that would bring it back.
CR has friendly battles that allow clan mates to fight each other. This would be a great idea for CoC. It would allow training sessions between more experienced players as well as general fun as clan mates compete for bragging rights, test their bases, etc.
Along the same lines, local clan tournaments, essentially a clan war within a single clan, would also be a blast. It could be broken up into teams of 5-10 players, King of the Hill, Elimination, or other games. This would be good for the social aspects of the game as clans could gather for a weekend torunament, watch each other battle (maybe have the leader moderate and set up matches), and chat while the battles are underway.
Since these would be controlled at the clan level, it would expand what being in a clan is all about.