#1 Nigel McGuinness
WWE faithful may know him as an NXT commentator, but the Ring of Honor fan base regard him as one of the greatest champions in the promotion's history. McGuinness is one of Bryan's greatest rivals you didn't know about.
Bryan and McGuinness have more alike than you imagine. McGuinness was also forced into an early retirement after he was diagnosed with Hepatitis B. However, the former ROH Pure Champion recovered successfully and has stated in the past that the only thing stopping him from returning to the ring is a WWE offer.
This statement was made back in 2013 when he was still a part of ROH, and now with him being in WWE, the possibilities are real.
Bryan and McGuinness' battle for the ROH title are the stuff of legends and the WWE Universe ought to get a chapter themselves. Ideally, Bryan should be the one who retires McGuinness if he wrestles in WWE -- which shouldn't be for long considering he's 42 years old.
However, an unknown rival fulfilling his dream of working a match in a WWE ring while also honoring a long-time friend's career does sound like a great storyline.