It's no secret that WWE has high hopes for Roman Reigns. He's probably going to be a main eventer at some point this year or next year. In some ways it can be argued that he's already there since he main evented two weeks of house shows in Europe with Randy Orton recently. He's not officially "the man" yet, but it's coming. The logical thing to do is have Reigns win the Royal Rumble and then the WWE Title at WrestleMania 31. It may not happen if they decide to go in another direction, though, so maybe he gets the WWE Title before that. If that's the case and he's free for WrestleMania then putting him up against Undertaker makes a lot of sense. It would be an awesome sight to see Reigns in a match against Undertaker, which would probably see Reigns win since Reigns would go into it as a babyface. They could do the ending where Undertaker shakes his hand or hugs him to put him over too. There might be other plans for Reigns at WrestleMania next year if he's going for the WWE Title or if The Rock wants to work with him, but if not then The Undertaker is a good choice.