(Spoiler Speculation) Bran

It probably has been here before, but I could find exact quote using search. (Maybe I am just no good at it.)

But rereading SoS this caught my attention: ‘’No,’’ said Jojen, ‘’only a boy who dreams. The greenseers were more than that. They were wargs as well, as you are, and greatest of them could wear the skins of any beast that flies or swims, or crawls, and could look throught the eyes of the weirwoods as well, and see truth that lies beneath the world.’’

So I believe that warg could take control of a dragon, but somehow Tully/Stark blood line seems less potent than Stark/Targ (if R+L=J is actually true)