I agree with a lot of the ones you guys mentioned, so I'm going to add a few old school names that I felt were held back.
Rick Rude could have been a World Champion in the WWE in the late 1980s and early 1990s. He had the look, the mic skills, and the ring skills. In the 1980s, he carried the Ultimate Warrior to his best matches ever, and held the IC title during a time it still meant something. He eventually left WWE and became the number two heel behind a red hot Big Van Vader, and totally destroyed the Mid Card scene for the better part of his WCW career. He even got a World title run (The International Championship was considered a WHC) finally. Its a shame, Hogan never main evented against him, nor did Savage ever main event against him for the title. Does anyone remember his feud with Jake The Snake? That was gold!
Another one the WWE dropped the ball on in the 1980s was Ricky Steamboat. He came of the best match in WWE history against the Macho Man, only to drop the title a few months later to the Honky Tonk Man. Yeah, Steamboat was set to have a long run with the title, and could have possible went further if he hadn't had a disagreement with Vince. Steamboat's wife was Pregnant and he asked for time off. WWE management wasn't pleased so they had him drop the title and his push was halted. This is also evident if you see the way he was booked at WrestleMania VI. Flair mentioned in his book, that Steamboat placed his family before wrestling and felt as if his wife (who wasn't big on the sport) held him back from reaching his true potential. Easily Flairs best opponent seeing as they have several five star matches together.
WWE could have done a lot more with Shane Douglas, even with his Dean Douglas character. However, this may have been mainly due to conflicts with the Cliq. Some of the Cliq said he was lazy in the ring at House Shows which caused problems, and I've heard a few other stories too. He really took off in ECW and could have been a great part of the WWE in the Mid 1990s when they were hurting for top heels.
Tatanka could have had a much bigger role but WWE screwed it hard with him. Who goes on a two year undefeated streak that leads to no where? Also, why have it ended by a guy that just got to your company and really didn't have that much heat? Those types of rubs should be saved for a heel that has heat and needs a push to get over the hump. He was supposed to win the IC title at WM (, but the Cliq stopped that at the last moment also. Kind of a common trend in the mid 1990s. Also kind of shit, since most of those guys jumped ship to WCW on the company when they were needed.
: I don't see how this matters. He's still entertaining as fuck and gets huge reactions from the crowd.
