#4 Sting
Sting is arguably the biggest star to have made his name outside WWE. Ever since WWE bought WCW, it was everyone's dream to watch him in WWE. The wait was long but at Survivor Series 2014, Sting did make his WWE debut.
However, during his bout at Night of Champions against Seth Rollins, Sting suffered a legitimate neck injury. The injury eventually forced him to announce his retirement during his Hall of Fame acceptance speech. It was a decision that was necessary but one that left many fans disappointed.
This deprived the fans of seeing him face The Undertaker. Sting has claimed that facing Undertaker is his "unfinished business" in professional wrestling.
Everybody wanted this match to happen, however, with Taker almost retired (following his match with Roman Reigns at WrestleMania 33) and Sting completely retired, we might not see it happen.