#3 Baron Corbin

After getting destroyed by Seth Rollins at the Extreme Rules pay-per-view, Baron Corbin will be looking to get on someone else's nerves now. Roman Reigns should be a good choice for Corbin to feud with.
Both guys have nothing to do currently and can have a short rivalry in that time. In addition, Reigns and Corbin have faced each other several times, so there is a certain chemistry between them. Reigns and Corbin went head to head in many matches last year, during the rivalry of The Shield and The Dogs of War.
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Vince McMahon has been trying to book Reigns in feuds against wrestlers who have a lot of heat from the fans, so Corbin should be the perfect option for him. Roman Reigns has also been getting good reactions from the crowd, so we can have a traditional face vs. heel feud to fill SummerSlam's mid-card.