Roman would have been heel if I were booking. I don't know how I would convince Vince of this, but I would do it.
Hell, I would have turned Roman when The Shield split.
Reigns should have been Diesel to Rollin's Shawn Michaels. He could have generated this massive heat by being Rollin's muscle and a mercenary of HHH who just beat the **** out of everybody who challenged Rollins.
Just face after face challenging Rollins getting ended by Reigns. This eventually starts getting Reigns some face cheers and Rollins starts to get cocky and think he can do it all by himself which culminates in a near loss to Cena after Reigns saves him one more time.
At this point Rollins gets upset because he feels he would have won anyways and the night after TLC they have a tag match against the top top 2 faces and after Rollins starts shouting orders Reigns walks out leaving Rollins to the wolves.
After this HHH patches things up and gets them on the same page while announcing a Cena/Rollins rematch at Royal Rumble and enters Reigns into the rumble as insurance.
Rollins almost loses the title to Cena, but once again, this time at the behest of HHH, Reigns interferes again costing Cena the match, once again angering Rollins. Later that night Reigns wins the Royal Rumble match, officially starting the feud.
I realize that a feud like that would take about a year to build properly, but it would have been way better.