Opinion: 2 possible ways in which Roman Reigns could finally turn heel by

#1 Turn his back on Seth Rollins

Attacking Rollins would be a classic heel turn for Reigns.

It would be the classic case of 'what goes around, comes around'. Rollins betrayed his Shield brothers in 2014 when he pledged allegiance to the Authority. Reigns could do the same by taking out the current Universal Champion. This ideally would be in a high stakes match where he would cause Rollins to lose the title to set up a feud. This would make booking sense as well since the BeastSlayer is currently booked as a babyface.

Reigns would use the moment to claim that he was tired of being outshined by Rollins - ever since The Big Dog returned from leukemia, he has not been booked nearly as strongly as Seth Rollins. To make things personal, Reigns could use weapons in his onslaught, cementing his heel status. This would allow the WWE Universe to see Reigns in a new light, as he has not been a pure villain since the initial run of The Shield.