#2 Shinsuke Nakamura wins WWE Championship
There is a strong likelihood that the primary reason Shinsuke Nakamura failed to capture the WWE Championship at WrestleMania was that the creative team thought it wouldn't go over well if he were to win it as a babyface.
With the Greatest Royal Rumble event being on the schedule, WWE might have figured that a three-step approach would be better, wherein he would lose at WrestleMania, turn heel and then win it just a few weeks later.
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Effectively, he ends up with the title just the same, but as a heel, so it avoids the chance that it comes off as trading a more popular star in AJ Styles for an unsuitable replacement if he were to have stayed a babyface.
Out of all the titles which could change hands on this show, this is the one most likely to happen, save for the Raw Tag Team Championship, of course.
Don't sleep on this being an uneventful rematch. It could have lasting effects on the SmackDown brand for months.