#4 John Cena
This man needs no introduction. He has already held the World title 16 times, tying Ric Flair's record, and I believe he's going to set a new record soon by winning the WWE Championship for the 17th time.
As you already know, John Cena will only work on major PPV's in WWE, same as The Undertaker, therefore I think he will win the WWE Title at one of the big four PPV's.
Considering all this, I think John Cena will probably return around next year's Royal Rumble PPV to set up his match with Samoa Joe at WrestleMania 36 for the WWE Championship. He might even win the Men's Royal Rumble and then challenge Joe at WM, possibly winning the title from him.
That being said, this could only happen if John Cena's schedule is clear at that time because if he wins the Royal Rumble match then he needs to appear regularly on WWE program for almost three months.