At this time, the WWE is reportedly dragging their feet in regards to signing Sting. The word is that WWE isn't in a major rush to sign Sting because they aren't looking to use him for WrestleMania 30. Sting was hoping that a match with the Undertaker would happen at this year's event.
Over the past few years, the Undertaker's WrestleMania opponents would meet up at The Undertaker's private gym to work out their match. Since Sting lives in the Dallas area, he is said to be willing and able to do this.
Most still expect Brock Lesnar will be Undertaker's opponent this year, but there has been a worry that Lesnar might not want to spend the extra time to work out the match in advance.