I think it should be Brock Lesnar. I know everyone thinks he will facing Undertaker again at WM32, but I'd rather he challenged for the title, leaving The Phenom free to face Sting in their final matches.
This is how I'd do it...have a heel member of The Authority hold the title going into WM, (maybe Rollins in his second reign), with Triple H and other faction members doing all they can to protect their investment, having J&J Security, Kane etc all appear in the Rumble, trying to eliminate Lesnar (who entered early).
Brock is left alone in the ring, waiting for entrant number 30.....AND IT'S TRIPLE H HIMSELF!!! They go at it for a few minutes, it looks like Triple H is going to eliminate Brock, but then a huge F5 takes Trips over the top rope and he crashes through a table that had been set up at ringside earlier, giving Lesnar the win and sending him to WrestleMania to face Rollins for the title.
What do you think?