Or you can just have Lesnar go over and Rollins cash in so you don't ruin Reigns' first title with a crowd that isn't happy. Or, you can bury the match in the middle of the card and end the night with Bryan, Ambrose, etc. going over who the crowd will be super hot for.
I've had a lot of little ideas running through my mind that don't have enough cohesion. Like you said, a double turn where Reigns goes heel and Rollins becomes a face should definitely be on the table.
I don't have an exact plan for it, but the biggest catalyst to all of this would be Cena joining the Authority. There is a certain perfection to it if it's done right. Think of when Rocky joined the Corporation. The fans were finding him stale, and he found a new gear to his character that made him interesting again, and translated well when he went back to being a face afterwards. Rock didn't need the wins, and Cena is far removed from needing them to stay "over." The symmetry is beautiful.
So, why would Cena join? That's the plot device I can't quite put together. It's certainly a bit of if you can't beat them, join them, but it seems deeper than that. There's some elements creeping into Cena's promos where he seems to be acknowledging the fans mistreatment of him. The Raw before RR I saw some of this. He can claim they blamed him for getting three guys fired even though he went out of his way to protect them all and get their jobs back. He can talk about how no one has his back any more but he knows the Authority will do whatever it takes to make sure they all stay on top. Cena joining opens up Ziggler/Ambrose/Bryan/Sting vs. Cena/Rollins/Rusev/HHH mix and matching, which could overshadow Lesnar vs. Reigns for a WM main event. Personally, I'd go for Ziggler/Bryan/Sting vs. Cena/Rollins/HHH to get things going, and then build from there.
If you want to move Rollins to face, him turning on the Authority for taking in Cena when he is "the Future" is more than enough reason for them to have issues with each other. Reigns could move to heel without going into the Authority itself, and you could build towards the inevitable Shield three-way dance at SummerSlam independent of the Authority stuff as they continue their program with Bryan, Ziggler, Sting, and even Ryback if need be.
Yes, I know they'll never turn Cena heel