Role Ending Misdemeanor: His career was littered with incidents of drug use and drunkenness (it's no coincidence that he used to tag-team with Scott Hall), so it's no surprise that his drunken brawl with Brock Lesnar on a plane ended his second WWE stint.
What Could Have Been: Often called "the best wrestler never to be WWE Champion", Hennig's career was plagued by constant injuries which kept him from being used by WWE to his full potential.
There was a period of about a month at the end of 1989 where the plan was for Hennig to go over Hulk Hogan at WrestleMania VI to get the WWF title. Popular rumor is that Hogan himself nixed the idea.
Before his death, the idea was floated about him possibly taking the NWA world heavyweight championship from Jeff Jarrett.
Curt barely dodged a bullet - Terry Taylor was also considered for the "Mr. Perfect" gimmick. When Hennig was chosen for that gimmick, Taylor was given the "Red Rooster" gimmick, which permanently destroyed his career.