#1 Brock Lesnar is the special guest referee
The only thing interesting about an otherwise stale feud between Baron Corbin and Seth Rollins is the addition of a special guest referee that will be chosen by Corbin. Corbin and Rollins have been feuding for weeks over the Universal title, but that's almost secondary to the main storyline between Brock Lesnar and Seth Rollins.
Ever since Lesnar won the Money in the Bank briefcase, a huge target has been looming on Rollins' back. Corbin is just serving as a placeholder challenger for Rollins as the Universal Champion's rivalry with Brock Lesnar is expected to continue through SummerSlam. Lesnar has teased a cash-in on many occasions, but he still hasn't cashed in on the champion.
Corbin hasn't announced his special guest referee, but there's a decent chance that it could be Brock Lesnar. After all, Corbin helped Lesnar win the Universal title last November against Braun Strowman. If Lesnar does become the referee, Seth could lose his title to the Beast.