If you're gonna bring in Sting, bringing him in just to lose to the Undertaker right away isn't the best move. Sting needs to show up sometime after WrestleMania 30, and be built up over the course of the next year to face the Undertaker at WrestleMania 31. Yes, he is an established WCW star and a legend in the business. But he also needs to be established in the modern WWE. Promoting Sting heavily on the WWE Network is a great start, they need to use history to their advantage while at the same time creating their own.
There are other big matches that Sting could have in the WWE before ultimately facing Taker. I can imagine a scenario where Randy Orton comes out on the RAW after WrestleMania 30, bitching about how he lost his Title (to Bryan hopefully) and saying he wants a shot at him. Then the entire arena goes dark, and a spotlight illuminates a figure standing in the rafters. It's Sting. RAW goes off the air.
With that segment right there, you introduce Sting in an impactful way and set him up in a nice first feud with a guy who is a solid worker. Orton can say that he used to be called the Legend Killer for a reason and that he will kill the legend of the man they call Sting. You do Sting/Orton at a PPV, and Sting goes over. From there you can move into a feud with Sting against the Authority, similar to when Sting feuded with that other faction...
Sting vs. Triple H is another big match you could do, possibly at SummerSlam, and would follow logically from that angle. Once again, Sting goes over. Triple H doesn't need to win.
Doing Sting vs. the Wyatt Family is also an option, if you wanted Sting to give the rub to an up and coming guy. It would be possible to put Wyatt over while still keeping Sting looking strong.
The last Sting match I would do is Sting vs. John Cena. The current face of the WWE vs. the face of WCW/NWA. Ideally, this match would take place around Survivor Series, and I think it would be awesome to see. Cena is a proven big match performer, and the crowd would be hot for this match. Sting beats Cena and as he is celebrating in the ring, the arena goes dark. A spotlight illuminates a figure standing in the rafters. It's the Undertaker. The stage is set.
Undertaker vs. Sting at WrestleMania 31, Taker goes over but it means so much more because Sting has been built up in the WWE over the last year. The build-up to this match alone would be so fun to watch, because they could do a lot of cool symbolism involving the two characters. Imagine segments where the Undertaker is in the ring and a video plays on the titantron showing a tombstone that says 22-1 with a crow perched atop. Or a segment where Undertaker does his signature kneeling pose in the ring while Sting looks on from above. You get the idea.