If the "opportunity" you speak of is a WWE/WCW/NWA World Title run, then the following apply for me...
Jake the Snake
Ted DiBiase
Mr. Perfect
Mike Awesome
Lance Storm
Test
Sandow
Barry Windham (not the NWA run when it didn't mean anything either)
Rick Rude (same as Windham)
Goldust (seriously should have been WWF World Champ after his controversial IC title reign)
Muhammad Hassan (NUCLEAR heat as the champ would have made for great TV)
Davey Boy Smith
Jake the Snake, Dibiase, Rude and Perfect all got screwed by the fact that no heel really had a shot at the belt once Hogan emerged. I don't recall if Macho was truly a heel when the Mega Powers exploded, but he never registered as one to me as a kid until he picked up a Sherri as his valet. Seems like anybody with upside got stuck behind hulkamania back then, even though it would have been better for everybody if they had developed one of those guys into a legitimate threat.
Mike Awesome wasn't that great, he just seemed larger than life wrestling midgets.
I'm surprised Lance Storm didn't have a lengthy mid tier run, multiple IC reigns, tag titles etc. He seemed perfectly suited to that kind of role, was he just crowded out by all the talent accumulated after the merge?
Test seemed like the prototypical lead guy. Especially with the brand splits that happened, he seems like a guy who should have been the next Sid, instead he's not even equal to Crush in my opinion.
Windham got fat and complacent, his longevity is testament to his skills, don't think he deserved better though.
I think Golddust hugely outperformed his apparent potential. Dustin Rhodes was such a lame WCW attraction, to take a sharp left turn and stretch 20 extra years of tv exposure out of his giant frame was a huge accomplishment. Big belt or no, that guy had a great run.
Bulldog had a great run. He was never the best performer anywhere he went, so being a long term second fiddle seems about right for his talent.