Whether Roman Reigns turns heel or stays face, a big showdown with John Cena seems inevitable. The company has spent the past year trying to make him into their new top star, and a battle with the old/current one can't be avoided for long. Turning Reigns heel would give a match with Cena a new dynamic. Instead of a program based on respect, they could have a feud based on anger and revenge. That typically works better when building to a high-level match. Going even further, fans might actually care more about the match if Reigns were a heel - if both men were babyfaces, they could easily end up booed out of the building at SummerSlam or WrestleMania. Strangely enough, Reigns being a heel may rally the fans to his side in a match with Cena, and that reaction would be preferable to outright disdain.