#2 Sheamus
Sheamus has been teasing a change in character for some time now. While it does seem like he will return as a referee, I wonder if there's something else afoot in this particular scenario al. Sheamus and Roman Reigns size up well and they have had some battles in the past as well.
We know that The Bar is no more and that Cesaro is part of the singles division right now. As good a worker as Sheamus is,as he is not Cesaro in the ring and may need a more storyline driven approach (with special emphasis on the 'driven' part). He would be a really cool contender for Roman Reigns down the line.
Maybe Roman Reigns mistakenly takes on Joe at WWE SummerSlam, not believing that Samoa Joe is innocent. But then it is revealed to be Sheamus, who interferes in the match and confesses his crime.