For a game centered around kings and kingdoms, there is a serious lack of Royal Regalia involved. My suggestion is to add items that only kings can obtain.
An example could be something like this: You can attach a Royal Sapphire, a Royal Emerald or a Royal Ruby to any tier helmet and it would give the same bonus as that helmet normally would, but it would take on the appearance a variety of different crowns based on what kind of royal gem you used. You can only get these royal gems if you are a king yourself, and you can only get them from long adventures. It could be entirely cosmetic, or it could add some kind of benefit. You could also make it so that you can add these gems to other pieces of equipment as well to make a full royal outfit to suit your cosmetic needs.
Every part of this is up for discussion other than the part that there is a very distict lack of Royal Regalia in this game.