#2 & #1 Brock Lesnar and Roman Reigns are instant favorites if either of them lose their Title match at Royal Rumble
Keep an eye on which Title match occurs first at the Royal Rumble. It would be strange for either the WWE or Universal Title match to open the pay-per-view. However, only four matches are currently announced.
Reigns is so used to closing every event that his match could go on early in the night. If he loses, it sets The Tribal Chief up to enter and likely win the Rumble. It wouldn't be all that exciting due to his utter dominance of WWE over the last two years.
The officials seem to be most comfortable with either Reigns or Lesnar as Champions. Reigns winning the Rumble would also be an excuse to have him challenge Lesnar if the Beast Incarnate retains at the event.
The match doesn't need a Rumble win to be booked, but WWE still likes that avenue of storytelling.
If Reigns or Lesnar loses, look for that person to enter and likely win his second Rumble. The Tribal Chief is the golden boy, so he has the best chance of winning if he enters.