#6 John Morrison
Tough Enough III winner and former WWE Intercontinental, Tag Team, and ECW Champion John Morrison was once set for a big main event run in WWE.
He had a great re-debut in 2005, forming one of the best WWE tag teams of the last 15 years with Joey Mercury as MNM, and later had a solid upper mid-card run where he was a 3 time Intercontinental Champion, but he was always meant for more.
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Although he did become WWE ECW Champion in 2007, the belt was devalued by the time he won it and wasn't really considered a Heavyweight Championship anymore.
Morrison had proven he was could be a World Championship contender back in 2009 however, thanks to several excellent matches with Jeff Hardy, CM Punk, Edge, Chris Jericho and more, and even challenged for the World Championship on occasions.
After a year of stops and false starts in 2010, Morrison entered 2011 challenging former partner The Miz for the WWE Championship on the first RAW of 2011, and gave a great performance. He would challenge for the title again in a Triple threat Cage match in the Extreme Rules main event, and although coming up short again, still gave a great performance. Following this, things took a very bad turn.
Morrison spent several months in lower card matches, often on the losing end. His last few matches in WWE were almost squash matches, and he departed WWE at the end of the year due to needing time off for injuries to heal. He elected not to return to WWE, and has since lit up the Indy scene and had a great run in Impact Wrestling, where he is the current Impact World Champion as of this writing.
I think WWE should have utilized him much better in 2010 and pulled the trigger on him as a main event player then more than ever.