Wow, so many awesome choices. Goldust and Hassan have always been curiosities in that they've never held the big title but both have been very highly over at their peak. hell, Goldust is still over, but i don't see a title run happening this late in the game. Then again, I never saw Jerry Lawler as number 1 contender at age 60, so....
If I had to pick people not already on the list, I'll try this bunch:
JYD: I don't recall him ever holding a title in either NWA or WWF. He was certainly over enough to hold the big belt, but I think he would have been perfect for a midcard title.
Tajiri: One of the few ECW alumni that managed to come to the WWE and make an impressive name for himself without cashing in on the invasion angle or the ECW brand that came years later. Well over as a heel or face without saying a word, and with William Regal in his corner he was golden. He held midcard belts, but was more than capable of holding the big one. I know some would complain about his size, but if handled the same way as Mysterio the audience could easily buy it.
Hardcore Holly: Another man who took a crap gimmick and turned it into something special. Was great in the ring, respected in the back, and was more than over enough to make for a solid world champ.
Val Venis: His gimmick sort of lost ground as the Attitude era was waning, but his wrestling never did. during the Attitude era it could have worked.
Mr. Perfect: Yes he held titles but never the big one, and that is truly a travesty. I'll go one further and say that he, Ravishing Rick Rude and Jake the Snake Roberts were all great talents that in their prime should have held and feuded over the world title.
Roddy Piper: A completely missed opportunity if there ever was one. He feuded with main eventers convincingly, was hated in ways few performers could match and never broke character.
Man this list just keeps getting longer. I'm sure more will come to me...