Great thread.
I'm sorry but I think many of you are missing one key thing about Sting:
Besides limited ability (age-wise), he's not known to the WWE audience. The IWC and long-term fans will know who he is, yes, but what you need to realise is that 50% of the WWE's audience are kids (I think it was Cole made that point recently). I'm not saying the WWE couldn't build him up as a legend, I'm not saying he couldn't have a successful run. I'm saying he won't have the immediate impact and recognition many of you think he will. Further, a Sting/'Taker WM match has the potential to be a terrible match. Sting is no HBK or HHH. That's the quality of worker Taker needs in his current condition to get a fantastic out of him.
Imo, and I concede that one day I could be proven wrong, Styles will never be a Main-Eventer in the WWE let alone a household name. He just doesn't have it in him. He's a very good in-ring competitor, I won't deny that, but he just doesn't have "it". He doesn't have star quality, as much as you may like him he just doesn't possess the appeal that other workers possess. He may be your favourite wrestler but that doesn't mean he can or will connect to a wider audience. Eventual Upper-Midcarder for the WWE is the best Styles could hope for. I think he'd flounder in the WWE, tbh.
Roode. Believe it or not I think one day he could do great things in the WWE. However, contrary to what many of you think, he's nowhere near ready to Main-Event for the WWE. I'm not a TNA hater but I do think there's a distinction between what it takes to ME in TNA compared to WWE. You have to tick a WHOLE lot of boxes and appeal to such a wide range of people in the WWE. TNA's audience, one could argue, are all of a much more similar demographic than WWE's are. It's the same for Styles, what you may like the WWE audience may not be impressed by.
I'm not saying either Styles or Roode couldn't be successfull, I'm saying they wouldn't have the immediate impact TNA fans think they'd have because they're familiar with the pair.
Kurt Angle. He could have similar problems as Sting, recognition wise, but it'd be far easier to educate the audience about his history, imo. He's also a MUCH better worker than Sting. He's got one part time WWE run left in him I think. While one shouldn't give too much credence to rumours, some have speculated that Angle could be one of the self-medicating breed in the wrestling business. If that is the case he might not be able to perform to standard without pain-killers. Maybe he's got no issues and the rumours are false but if they're true it could be a huge factor in him avoiding the WWE.
I'd echo the view of Anderson as over-rated. Not a bad performer.....but not a great one either. He's had more than enough opportunities to shine in both companies so I don't think he'll ever be a huge asset to either company. Comfortably performing in the midcard/upper-midcard for the rest of his career, wouldn't have too much impact in the WWE.
Matt Hardy: As someone else said, a sober Matt Hardy is much more use to TNA than WWE. Those who say he was never too over in the WWE towards the end need to rewatch his last Rumble entrance and the MITB match that involved Christian. The crowd can emotionally invest in him because he's been around so long that the fans have been on a "journey" with him so to speak, that was a huge factor in Jeff's rise also. Not that I think WWE would ever give him that opportunity again.
Kevin Steen/Aries/Etc, would all be starting from scratch in the WWE. They'd go the way of Daniel Bryan: over with the IWC who would complain about EVERY loss on the way up the card. For that very selfish reason alone I hope they never go the WWE....I couldn't listen to the same rants DB inspired on this forum!
Jeff Hardy. Like it or not he has widespread appeal. As JR said it's a very rare breed: a performer that appeals to men, women and children equally. The audience are not only familiar with him, they LOVE him. Those who say he wasn't quite at Cena's level should note that towards the end of his WWE run, 8 of the 10 best selling items in the WWE store were Jeff Hardy items. To this day I've never seen a worker get even close to that, not Cena, not Punk, nobody. He's the only worker I've ever heard of that got offered a WWE contact with no wrestling commitments...just the right to sell his merch. They did that for a reason. Whether you like him or not he brings the most to the table out of that whole list, by a mile. I concede that his stock would rise, immeasurably, when he finishes his probation in a little over a year I think.
You're all missing a key person: Batista.
I don't like Batista, at all, but he's proven to be over with the non-IWC audience and could make an instant impact in the WWE. For sheer fact of Hardy's probation issues, Batista could be the best addition out of anyone mentioned in this thread. The WWE need another Main-event full-timer. Batista is a ready made Main-Eventer.