For what it's worth, it was a Smackdown guy that won the Royal Rumble the past couple of years so I'd suspect it's RAW's turn this time around, hence my Seth Rollins prediction.
If I were to pick a dark horse, I'd probably go with Drew Mcntyre but from a story line perspective, he doesn't quite tie in....not as much as Rollins anyway. I mean, Lesnar vs. Mcntyre is surely a Vince type of match and something new and fresh (quite frankly, I would like to see it myself) however Lesnar versus Rollins has a better ring to it at the moment so, yeah...sticking with Seth...