Daniels, AJ, Joe, and the MCMGs have all had try outs with the WWE already and as you can see none of them made it. Even though they've made names for themselves they still wont be that big a deal in WWE just because they wont get pushed.
Any of the tag teams, including Beer Money inc., would just be a waste since WWE really never pushes the tag team division. Any of the cruiserweights would also be a waste since WWE has strict rules on high flying. If you remember even the shooting star press was banned until Bourne came along.
I think alot of the TNA originals could be big names for WWE and the future of WWE if they were to be pushed right, but Vince only likes the Batistas and Kozlovs so if you are a smaller guy and not apart of the WWE during the attitude era you're pretty much screwed.
As far as Nash, Vince actually tried to get him to come back last year and offered him a contract, but he wanted Nash to color his hair. Nash refused for many different reasons, mainly because of the stroke he has in TNA which honestly if he got back in friendships with HHH and HBK he'd probably have more say in the WWE aswell.
I think Sting is the biggest one from TNA who could benefit WWE and hurt TNA. TNA gained a pretty good number of supporters after they signed Sting and like someone said there's plenty of matches people would want to see if Sting went to WWE.