May as well throw my hat in the ring... and go in a vastly different route. This is based on the premise of WWE calling an Audible and sending Daniel Bryan to the championship match.
Leading up to Fast Lane, I take the budding angle of Reigns not caring about friend or foe and actually have him brush up against Ambrose on their respective ways to their own feuds, and generate some strife. It wouldn't take too much. Ambrose calling Reigns a coward for not having his back all those months ago. Reigns calling Ambrose an idiot for taking his eyes off the prize and losing his balls because he's now chasing titles instead of revenge. But the key thing is to break the last remaining bond in the Shield.
Fast Lane comes and the IC Title match between Dean Ambrose and Bad News Barrett comes with a shocking twist- Reigns comes in, spears Ambrose, and screws him from winning the title.
Later on, in the Main Event match against Reigns and Daniel Bryan, Ambrose comes in, somehow manages to screw Reigns without getting the match thrown out, and steals away the #1 contender spot.
What you get in the next couple of Raws and Smackdowns are the bad blood this mutual double-crossing has invoked. Run-ins, backstage beatdowns, angry promos(something I think Reigns could do if they don't micro-script the fucking things) and just outright animosity, something that the Authority would take GREAT delight in seeing. And not just for two babyface heroes taking themselves out of the equation.
It's also because Seth Rollins would enter the scene at this time, pouring sweet honey down both of their ears. Since Roman and Dean want each other dead, they are suddenly BOTH willing to sidle up to the Authority and Seth in particular to achieve that end. You can really work with Seth trying to 'Stockholm Syndrome' the idea of why he betrayed the Shield in the first place and why the Authority was a better move for him. You want him coaxing the both of them to the Dark Side.
At the same time this is happening, you have both Dean and Roman appealing to HHH about a grudge match at Wrestlemania. They both want the cell, and they both want to stop at nothing to see the other wrecked. But to do that, they need extra insurance in the cell... and Haitch has the idea.
So we go to the final Raw before Wrestlemania and we have Haitch in the ring with Dean and Roman as they quibble over their contract signing. But then Haitch has a surprise for the both of them. He calls out Seth Rollins, and informs him he has been added to the HiaC match, and shrugs off the inevitable protest, saying Rollins won't be in there as a competitor, but as the Special Guest Referee. After all, who better to oversee the final demise of the Shield but the man who started the ball rolling? (Backstage is another side to the story, as Haitch tells Seth in no uncertain terms that both of them will be sucking up to you to gain their favor, so you can screw who you want and the other will be indebted to you and the Authority. And in return, you take an easy night free of getting your head clubbed in, and leave you fresh just in case... a point to the Briefcase for emphasis.
Wrestlemania. 3rd to last match on the card. Hell in a Cell. Roman Reigns versus Dean Ambrose. Special Guest Referee Seth Rollins(who carries the Briefcase for no damn reason). They'll bill this something like 'The Fall of the Shield' or something equally melodramatic. Anyhow, the cell is lowered, all three members of the Shield are in the ring, alone(because Haitch is too confident this will go according to plan). And for the first fifteen to twenty minutes, it goes as normal as you'd expect. Roman and Dean beat the utter hell out of each other in a back and forth match while Seth watches and does a few heelish things, like secretly passing a weapon to one while the other isn't looking, or deliberately slow counts, or if rope rules are still in effect 'forgetting' the five-count because hey, this is the first time he's done this, tee hee!
But finally, Seth picks his spot, lays Dean low with a dirty move, puts him in a position to feed him the Curb Stomp while Roman cheers him on, selling the notion that he has gone full heel. Seth starts the run up and rope rebound to execute the finisher...
...only to run right into the Superman Punch.
That's right. A whole entire two month's worth of animosity and agony and feuding and sucking up and turning to the dark side... it was a complete and total ruse. All just to get their traitorous former brother alone in a cell with the two of them. The bell is rung, the match is thrown, and the Cell mysteriously cannot be lifted, as if SOMEONE wrecked the hydraulics mid-match.
Haitch and the Authority and anyone else will be rushing to the entrance to try and break their way in while Roman and Dean proceed to utterly payback Seth DOUBLE for what he did to them. You see him take a couple finishers from both guys, you see them put his freaking HEAD through that Briefcase(we've seen that freaking thing in Seth's hands so often I WANT to see that happen goddamnit!), and then, just for the coup de grace, just before the Authority can storm in... they make Seth eat a Double Curb Stomp.
This promises a major distraction for the Authority as they have to openly feud with Reigns and Ambrose as a result, it delays the MitB cash-in, potentially preserving a great Wrestlemania Ending in the process, and it gives the Shield feud a good and proper progression, making Rollins the hell-bent bastard wanting revenge, even if he has to subvert his masters in order to get that. Imagine that, Rollins betraying Haitch and the Authority to achieve his own ends- something that MIGHT be worth Rollins getting shat on at Wrestlemania.
As for Reigns, this does kinda subvert him from that Main Event hopeful, but with business settled to some extent you can always move him on up after WM and build him further. Or you could keep him with Dean and feud directly with the remnants of the Authority and keep him in juicy storylines as a means of getting him even higher with the fans.
(Thejock83, so sorry if I went and hijacked your thread. I figured it'd be commandeered as a general 'Reigns Wrestlemania Storyline' thread.)