OK, here's what they have to do for Mania with Reigns.
He's going to get booed no matter what, so he has to win that title as a heel to avoid Mania being a washout. At this point, there's no way he wins it and remains face in my eyes.
So, they should have HHH drop the title to Ambrose at Roadblock. At that point, HHH should demand a rematch at Mania, but he'd have to include the number one contender (Reigns) and Ambrose's scheduled opponent (Lesnar), making it a fatal-4-way.
At Mania, at the end of the match, Reigns and Ambrose should face off. HHH crawls into the ring; Ambrose goes for HHH, then Reigns low blows him into the Pedigree.
Then here's the twist - HHH invites Reigns to cover, Reigns wins.
It'd work as HHH is a transitional champ and only has it as Reigns wouldn't acquiesce to his authority from Survivor Series onward. If Reigns suddenly becomes the new Rollins, HHH is happy to have him as his champion.
That then sets up Reigns as a heel and Ambrose as a face as a stellar fued going forward, and will only ignite further when Rollins returns.
That's how they should book Mania.