#3 Shinsuke Nakamura
Assuming Roman Reigns does not actually win the Royal Rumble because of the Elimination Chamber being a Raw-branded event, that means someone from SmackDown has to do the deed.
It doesn't make much sense for Kevin Owens and Sami Zayn to be co-champions with the WWE title, so the smart bet is on AJ Styles retaining and keeping that belt until WrestleMania.
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Judging by the landscape of the blue brand, there really aren't any options for who could challenge Styles other than Shinsuke Nakamura, which makes him a prime candidate to win this year's Royal Rumble.
In the past, the Royal Rumble used to be a tool to promote someone from the upper-mid card into the main event, similar to what Nakamura's current status on the roster hierarchy translates to.
He's a babyface, meaning the crowd will accept his win much more than someone like Reigns, and the appeal of a Nakamura vs Styles match is in direct opposition to the mentality behind the Reigns vs Lesnar match—giving fans a bit of both side of the world of sports entertainment.
Nakamura winning means the "smark" audience gets their match at WrestleMania while the other main event isn't jeopardized and Vince McMahon gets what he wants, too.
If WWE merely ignores the boos they'll get and the decision to have Reigns win the Royal Rumble is made, rest assured Nakamura will still get his moment to shine in the match.