I'd like to see either Daniel Bryan, Roman Reigns, or The Rock win the Rumble. With the almost certainty that Brock is going to retain at the Royal Rumble, we can eliminate a number of stars from this list seeing that the Wrestlemania match will most likely be heel vs face. Therefore, Wyatt, Rusev, and Rollins probably won't win. Doubtful that Triple H or Sting will win because it looks like the company is steering towards HHH v Sting at WM 31. Highly doubt they'll let Cena win and face Brock again because that match will undoubtedly be overplayed (already kinda is). Cesaro has been put on the back burner so we can cross him off. No way they'll give it to Batista again after everyone's reaction to him winning it last year. Dolph is obviously on the rise, but a win at the Royal Rumble doesn't seem like the direction they'll be going in (look for him to win at TLC and put the IC title on the line at WM).
That leaves us with Bryan, Ambrose, Reigns, Orton, and The Rock. My thinking is that out of these 5, Orton is least likely to win because he'll probably be feuding with Rollins once he returns to action. Ambrose is probably the next least likely candidate, but it's still a posibility. I don't see his feud ending with Wyatt anytime soon, as they're arguably the hottest faces/heels atm. That leaves us with Bryan, Reigns and The Rock. If Bryan returns to action in time it's certainly a possibility that he'll be entered in the Royal Rumble and could be the one pointing at the Wrestlemania 31 sign at the end of the match (always found this corny lol). I think Bryan vs Brock would be a great match, even though the difference in physical stature is insurmountable. Reigns is definitely a good candidate to win at the Rumble after the huge push they gave him at the same event last year, but my only question is if he's ready or not. He's got the in-ring ability, but I worry about his mic skills. His promos just don't feel natural at this point and giving him Royal Rumble and Wrestlemania wins (Brock will most likely lose at WM since his contract will be expiring) could be detrimental to his progression. The last likely candidate to win is The Rock, even though I'm not sure if he's even expected to compete at Wrestlemania this year. Although I think Rock v Brock: Wrestlemania 31 would sell a lot of tickets/subscriptions, my only problem with this is that they'd be bringing back another part timer (no disrespect, I love The Rock) to win the Royal Rumble which kinda diminishes the abilities of the younger, more active competitors. There's a lot of ifs regarding the Royal Rumble winner, but in my own observations, it'll most likely be Bryan, Reigns (especially after winning Superstar of the Year last night), or The Rock.