As far as superseding Daenerys' claim goes, any child - male or female - of Rhaegar would automatically supersede her. That is, she's a daughter of King Aerys, while Rhaegar was the crown prince - he is her brother. In these kind of dynasties, inheritance passes from firstborn son to firstborn son - so a king's grandson would ascend to the throne instead of his secondborn son. I think Edric Storm is the only bastard he had by a highborn girl. While he didn't marry her, was was required to recognize the child as his - as opposed to say Gendry and Bara. ...which, yeah, is totally something Robb could have done.