#1 Steve Austin
The chances of this happening are almost zero. But, it would probably be the most epic moment in Royal Rumble history if Steve Austin made a surprise return at number 30, especially if it could be kept a secret beforehand.
Since retiring back in 2003, Austin has repeatedly said that he has no interest in wrestling again, with one of the main reasons being that he'd have to get back into insane shape to do the type of match he'd want to do, and he's just not interested in committing that kind of time to an in-ring return, as he has other things going on.
But, if he came back during the Rumble, he could come in as the final entrant, and get eliminated by whoever their top heel is a few minutes after he hits the ring. All he'd have to do is hit a few Stunners, throw a few people out, and then get eliminated by a guy like Kevin Owens.