Ruthless Aggression era:
>Chris Jericho, Booker T, and Rob Van Dam
Some combination of these 3 should have maintained their main event push during the early Ruthless Aggression RAW, Triple H didn't need to completely hog the entire scene. Although this is probably a mostly hindsight take since they were trying with Kevin Nash, Scott Steiner, and Goldberg. 2003 RAW was fucking grim.
>Christian
Post draft 2005 he could have absolutely been a top heel on Smackdown.
>Mr. Kennedy
He had chances but was on the receiving end of injuries, bad luck, and then some politicking at the finale of his run with WWE. It's a shame, he was entertaining.

PG Era
>Dolph Ziggler
Great look, great in the ring, good enough on the mic. Idk what they didn't see in him to not want to push him.
>R-Truth
His early 2011 run was great and he showed he had what it took to be in main event programs. Shame he regressed to the status of a comedy act.

Current Era:
>Drew Mcintyre
This one probably doesn't count as much since he's already a multiple time World Champ and Mania main eventer but her should definitely be in the World Title picture a lot more.
>Chelsea Green
She has it all to be a top woman in the company. Don't know what they don't see.