Dave Meltzer noted in the Wrestling Observer newsletter that The Undertaker vs. Sting will not be happening. There is no specific reason why other than Vince McMahon is negative on that match so it's not going to happen. Sting's status with WWE is that he is not considered to be a retired wrestler and there are plans for him. There is also a good possibility that Sting will go into the Hall of Fame in Dallas/Arlington. Sting lives near Dallas.
Daniel Bryan is not in the plans for WrestleMania right now since no one knows if he will ever wrestle again. Vince McMahon has the top of the card already figured out in his head. That includes the top six matches for the show but there's no word yet on what those matches are. Also, John Cena's match is said to be locked in but there's no word yet on what that match is.
The main event is going to be The Rock and Ronda Rousey vs. Triple H and Stephanie McMahon or they'll split them up into two singles matches. In that case then it would be The Rock vs. Triple H and Stephanie McMahon vs. Ronda Rousey. Of course, everything depends on whether WWE can convince UFC to let her do the match.