I like the concept of the gravity dragon quite a lot. That could be a very interesting magic. Making a feather powerful enough to leave a crater if dropped, or make a cannonball change direction in midair.

One concept that just crossed my mind would be a time dragon. Obviously due to what this kind of magic would be capable of, it would probably be the most difficult to use and have [b]serious[/b] limitations, but it could also make for an interesting one.