1- Roman Reigns taking out Cena while going for Bray
The friction between Shield and Wyatt Family is not a secret at all. The trios have been getting under their noses for a long time now especially Bray Wyatt and Roman Reigns.
So it wouldn’t be a surprise if Reigns decides to act as a distraction to Bray Wyatt during his big WrestleMania moment. Reigns could work out in any match (Shield Breakup match ideally) the WWE puts him into and come out during the time of this to get something on Bray.
However, things wouldn’t go as planned as Reigns could take out John Cena while going for Bray and thus costing the Cenation leader his match.
A Cena seeking revenge for an accident will be an intense character as well like him joining the Wyatt Family. Opening up a feud with Cena with shades of being a face will clearly benefit Reigns on the long run.
Bray will win the match as well giving him the much needed boost on the big stage. The company could then book him against another big name while leaving the stage for Cena and Reigns to fight it out.
Adding a title in between them after sometime will make things more intriguing and will simply establish both Reigns and Bray as main eventers.