#1 John Cena

If WWE is not ready to have The Fiend lose to Roman Reigns just yet, one of the best options for the Universal Championship match would be a man who has not had a match for the belt ever. John Cena has won 16 World Championships but has never challenged for the big red/blue belt ever.
If Cena were to have a match at WrestleMania this year, there isn't a better role for him to play than to put over the star he should have put over years ago. As you already know by now, The Fiend never forgets.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
This was one of the first acts of injustice committed onto the Bray Wyatt character of old. Cena beating him at WrestleMania 30, completely halted any momentum for the Eater of Worlds. A rematch six years later would allow Wyatt to get that win back in a major way.
What makes a Cena vs Fiend match even more compelling is the aftermath of it. How would Cena change? That alone makes for a brilliant story, with the Leader of the Cenation returning to debunk the effect of The Fiend only to get sucked in himself.
This would be an excellent match for WrestleMania, with Cena's pursuit for a 17th World Title adding to the layers of an already compelling story. If Cena does come back for a match at 'Mania, The Fiend would be one hell of an opponent. Let him in!