I've yet to see anyone discuss a Reign's heel turn, so I thought I'd be the first.
What I think they should do is give him a manager who's good on the mic. Now when the manager talks, him and Reigns are not to be in the ring, instead their promos are done at the entrance way. The reason for this is so Reigns can turn his back to the entire arena, while his mouthpiece talks for him. Reigns is never to speak. Let this go for as long as they can, months, even a year.
A mystique will begin to build with Reigns never speaking.
Sting was never bad on the mic, but he was never great either. Not on the level Flair or Austin. But what happened when he morphed into the Crow character? He didn't speak for over a year, and this mystique built up over it, even though we had 10 years of Sting's promos.
I like it, but let's go a step further with it and have him never wrestle, either. He can just stand there while other people talk for him and fight his battles for him. THEY'LL MAKE MILLIONS.