#2 Drew McIntyre
Despite not wrestling in WWE for over five years, CM Punk is still very over with the fans as a babyface. It would be a real stretch to turn him heel, should he step back in the ring. WWE will likely need a strong heel to play the foe to CM Punk's anti-hero, hero. Drew McIntyre's character has been very strong since he was "called-up" from NXT. He's been in and around the main event scene for quite some time.
Many people are speculating that it's only a matter of time before McIntyre holds the Universal Title. WWE management has been very high on the Scottish Psychopath and The Rock even chimed in saying Drew McIntyre is the "next big thing". If WWE sees him as a bonafide main-eventer, then doing battle with CM Punk would provide the rub to make that final leap.
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Win, lose or draw, Drew McIntyre is a heel that draws boos and legitimately intimidates fans with his look and presence. He's one opponent fans would love to cheer CM Punk against.
If WWE thinks of him as a future World Champion, then putting him in such a high-profile feud would be further evidence of such trust. As this character has proven, he can take a loss and still show up just as intimidating the next night.
There is only one person who might be a better foe than McIntyre and we'll cover him on the next slide.