Finishing off the Clan Boss
Since we often can't do very much damage when fighting the last battle against the Clan Boss needed to kill him, then maybe we should get an extra chest or an extra key. Some kind of bonus.
EDIT: And if the Clan Boss is already down to, let's say 1-3%, and someone else is already fighting him, then it should lock out anyone else trying to fight him until that person is finished. Then, if the Clan Boss isn't dead yet, someone else can choose to fight. Perhaps, when that occurs, it should have an indicator saying Fight/Battle in Progress or something?