#2 Bray Wyatt
Bray Wyatt has been entertaining the WWE universe with his pre-recorded Firefly Fun House segments on RAW and SmackDown since Wrestlemania 35. He is yet to make his in-ring return after his character transformation. WWE universe definitely does not want another Baron Corbin vs Seth Rollins Universal Championship match after this Sunday.
So WWE might book the returning Bray Wyatt as the referee to plant the seeds for a fresh Universal Championship feud for Rollins. A rejuvenated Bray Wyatt's fun-filled antics as the referee will definitely add some interest to the otherwise regular championship match.
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#1 Brock Lesnar
This may seem far fetched as Lesnar generally does not appear on B shows let alone as a guest referee. But Brock Lesnar's recent track record proves that he can surprise WWE universe at any point in time. Also, Brock Lesnar was on the receiving end of several brutal chair shots from Seth Rollins at Super ShowDown PPV.
So Beast Incarnate will be eagerly waiting to exact revenge on the Universal Champion. Brock Lesnar as the special guest referee will be the best possible option for WWE right now. It will make the contest intriguing and increases the uncertainty about the result of the match as Brock still holds the MITB contract.