#1 Heel Turn - Roman Reigns
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Just like the possibility of the New Day turning heel, Roman Reigns doing the same thing is highly unlikely. That doesn't mean that we cannot predict or suggest that him doing so might freshen things up for both Reigns and the WWE.
He's still in the "Honeymoon" phase of returning from his Leukemia diagnosis, so a heel turn isn't happening any time soon. But once enough time has passed, a turn would be something that most people wouldn't see coming. We've all come to expect Reigns to be the face the fights anyone and helps his friends. That might get old as it did during his pursuits of Brock Lesnar and the Universal Championship.
A heel Reigns would be a remorseless assassin that would instantly gain credibility as a bad guy. He's literally feuded with all the top heels in the company at given moments - Drew McIntyre, Braun Strowman, Lesnar, Samoa Joe, AJ Styles, Randy Orton, Kevin Owens. So what's to happen when he runs out of bad guys to vanquish?
Revisiting those feuds would be entirely stale and thus a huge change would be needed. I'm not suggesting a Bray Wyatt-level change. All they have to do is have him either attack Seth Rollins or whoever is the top babyface of the moment and there we go. A heel Reigns might be the best thing for his character once all of his other options are exhausted.
Did a WWE Hall of Famer predict John Cena's heelish ways long ago? More details HERE.