No Super Bowl appearances and not all that good of a record. The fact that Madden, Fouts, and Gibbs are in the HoF (they got there once they left the Coryell system) could be an argument against Coryell, too. I don't feel that he's the only innovator to not be in the Hall.
All that said, he has become a big legend. He probably fits the fame part of the induction, whether he deserves it for the other reasons like stats and performance, that is the big question.