#3 Matt Riddle
How could you not put Matt Riddle and Brock Lesnar in the ring for an exchange? With their mutual UFC backgrounds, it's the most natural thing in the world. Whether it would be left as a one off or a foreboding for something to come in the future, it should be a no brainer that Matt Riddle will enter the Royal Rumble match and take the WWE Champion for a scary ride.
Elsewhere, having Matt Riddle show up in the Royal Rumble match would be an acknowledgement of all the good work he's done over the past year and a half or so. Lest we forget that Matt Riddle was also on the NXT team at Survivor Series, where he eliminated no less a figure than Randy Orton.
Matt Riddle would add a lot to the Royal Rumble match. It would be foolish to not include him in it. Perhaps he could get revenge on Baron Corbin for eliminating him from Survivor Series along the way. There are limitless possibilities with him.