I'll go with the big five divisions
If Joe Lauzon were more durable, and had better defensive game he could very well be a LW champion. I actually think that he's like Shogun where I can see him beating everyone in the top ten but also losing to everyone in the top ten.
Another guy is Jake Ellenberger, if he had better top control and/or cardio he could be elite. Really if he drops down to 155 and loses his excess muscle I think he could win the belt.
If Michael Bisping learned BJJ and used it he could be undefeated at this point. I think a big reason he'll never be champion is his game is so limited and predictable.
Phil Davis needs to learn boxing badly as he has all the other tools to be successful.
Roy Nelson's biggest issue is that he's so slow. He has great cardio but his handspeed, footwork, and wrestling would all need work.
You know what else he'd be at 155? Dead.