#6 Booker T

This is a stab in the dark, of course, but no Royal Rumble would be complete without a returning legend or two, and Booker T stands out as one that has yet to have a big moment as a returning legend for a Royal Rumble in recent years.
Booker T is still employed by the company and it wouldn't take much for him to put on his boots and get to the ring on Sunday night. As soon as the words "Can you dig it, SUCKA?!" get announced, there would be an uproarious pop from the crowd in Phoenix, which is one of the things that makes a Royal Rumble match so much fun to watch.
We could see an amusing, crowd-pleasing spinaroonie or two, much to the chagrin of serious competitors like Andrade "Cien" Almas, Drew McIntyre, or Samoa Joe, before Booker T gets eliminated from the match.