Give the injury and loss of momentum to Roman Reigns recently, I'd just have him return and finish the long Authority feud that began the day after Wrestlemania 30.
I chose the Rock vs Brock Lesnar as my option. I have no doubt it would draw big-time which is why it has been rumoured so extensively. However, defeating Brock Lesnar is supposed to be a huge rub for someone, which is why Reigns is an ideal superstar for it(Ambrose is an option, but lMO, I am still on the fence with regards to that option).
Another option is Daniel Bryan, but that will depend on his fitness. The storyline writes itself anyways. He still has a rematch since he never lost the title, and Brock would be yet another obstacle in his Underdog journey.