The perfect booking plan for a Sting WWE run would be as follows:
Sting appears (without his trademark face paint) a month or so prior to Wrestlemania after it is announced that he will be inducted into the HOF. He cuts a promo about how he thought he'd never see the day when he stepped inside a WWE ring, etc. The Undertaker comes out and confronts him and challenges him to a match at Wrestlemania. Sting declines and reveals the reason he is declining is because he quietly "retired" after leaving TNA (not mentioned by name obviously)
Undertaker comes out the following week and cuts another promo where he reveals that he has "retired" following every recent Wrestlemania victory, but he is then lured back into the ring by the prospect of a new challenge and so far, every year that he has "retired" a new challenge has emerged. Undertaker says that Sting is that new challenge and if he declines then not only have the fans lost Sting, but they've also lost Undertaker.
Sting comes out next week to a mixed reaction (still minus his face paint) where he applauds Undertaker for his efforts, but he's still "retired" and it's going to stay that way. The following week Undertaker comes out to open the show and says that since Sting has declined his challenge there will be no streak match this year, and maybe never again. Undertaker then says that he will instead give the fans one last match and poses an open challenge to anybody (whoever is currently popular comes out and the crowd goes wild). During the match however, the arena goes black and when the lights come back on, Taker's opponent is laid out in the ring with Taker going for his usual tombstone pin (no referee) but with the addition of Sting (in full Sting attire and face paint) standing over him with a baseball bat - you can guess what happens next! Sting then reveals that he knew from the beginning that he wanted a shot at the streak, but he forced Undertaker to play by his rules, his mind games and now that Sting has that advantage - it's showtime!
Undertaker defeats Sting at Wrestlemania after a classic which could have gone either way.
Sting returns some time after Wrestlemania and wins a battle royal for a shot at the WWE Championship. The storyline is a Rocky Balboa'esque scenario where Sting is trying to prove that he has what it takes to be a WWE superstar. Sting then gets his shot at the WWE Championship, which he wins (bear with me!) and carries until the next major PPV where he puts over WWE's chosen choice (preferably a rising star) clean in what would essentially be Sting's retirement match.
So... We get the Undertaker/Sting mania match, Sting gets his chance to be a WWE superstar and WWE Champion and he then puts a future star over huge on his way out.