#2 Conor McGregor

It's no secret that, at one point, WWE was interested in doing business with Conor McGregor, but the two sides weren't able to come to an agreement, as WWE wasn't willing to pay the UFC megastar the amount of money that he was asking for.
It's also no secret that McGregor would be a fantastic fit for WWE if he came in with the right attitude. If WWE were to sign McGregor, they'd have to be willing to accept any potential baggage that comes along with the UFC star. But, in the end, taking the risk might be worth it. Undoubtedly, there's no one in pro wrestling, boxing, or mixed-martial-arts who is better at selling a fight than Conor McGregor.
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There would be a major size issue if McGregor ends up in WWE, especially if they do the match that everyone would want to see, which is Conor vs. Brock Lesnar. Obviously, it'd be a huge mismatch in terms of size, but it would be the biggest match they could do with the ex-UFC Featherweight Champion.