#4 Only real contender in the current roster
The SmackDown roster at this time is loaded with talent, and yet Daniel Bryan is the only real contender for the Championship at this particular point in time. Braun Strowman is locked in a feud with Shinsuke Nakamura for the Intercontinental Championship, which should play out at the Royal Rumble.
Similarly, Roman Reigns and Baron Corbin are obviously not done with one another and both men should settle their score during the course of the Royal Rumble match. Ali and Shorty G are both awesome but they have a long way to go before they get Universal Championship opportunities in my opinion.
You could always make the case for Kofi Kingston but he's one half of the Tag Team Champion at the moment and I'm guessing will also be a big part of the Royal Rumble match. That only leaves The Miz and Daniel Bryan, and you know the story.