The optional boss thing isn't a bad idea, but I believe that the issue is what will always happen with optional bosses. I.E. people will start skipping them as the bonus comes from completing the dungeon once a day. Now a good way to do it would be to add a strong incentive, something that comes to mind would be one of the dungeons from WoW where you had to reach a point in the dungeon and kill an optional boss in a certain time limit. If you did, it gave you a guaranteed drop mount. Something like that which is guaranteed that is on the way to the final boss is a good way to get people to do them.