One word: Excalibur.
If there was a need to balance dragons, this would have been better than introducing a new type (or better yet give steel the boost it needs and introduce both)
Steel types strength is in its durability, it's only weak to ground, fire and fighting, and few types have no weakened matchup to steel. Also, although in western culture Excalibur was a sword made renowned, it was in the process of selecting a King of England. Many of the killing of dragons with swords tales are frowned upon in eastern culture because most of the life giving forces of nature, rivers for example, were a part of the dragon themselves and the death of the dragon meant the destruction of the life sustaining resource, it's a mentality and cultural differential that must be considered and understood, it's actually a theme in the games themselves, ha! :D