Finn Balor Wins Due To Outside Interference
This is the most believable path to a Finn Balor victory. Brock Lesnar does not lose clean. Even Roman Reigns defeated him after Braun Strowman inadvertently distracted Brock Lesnar at SummerSlam 2018. Reigns had main evented four straight WrestleManias, won a Royal Rumble, won three world championships and defeated The Undertaker at WrestleMania 33 at that point.
There are two ways by which Balor can win due to outside interference. The first involves interference by Lesnar's WrestleMania opponent. One must assume that he will only lose the Universal Championship before WrestleMania 35 if he is in an even bigger match. That opponent costing him the Universal Championship is an excellent way to set up that match.
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Indeed, there is a past precedent; Goldberg cost Lesnar the WWE Championship at the Royal Rumble in 2004 in order to set up their first WrestleMania match. The second way involves interference by Luke Gallows and Karl Anderson. The history and friendship that these men share is well known. Finn Balor could win if they help him.