#4 WWE Universal Champion Roman Reigns
The idea of a WWE Royal Rumble showdown between Adam Pearce and Roman Reigns was something that made sense in terms of Reigns' characteristics as an onscreen persona.
The Tribal Chief has established himself as somewhat of an authoritarian mafia don who doesn't like being dictated by other authority figures. The most sensible creative direction for someone whose authority is being challenged is for that person to take out the opposing threat. In the context of Roman Reigns' storyline, Adam Pearce is the opposing authority, which is why The Tribal Chief targeted him on the Road to WrestleMania 37.
However, Pearce seems to have tactically outsmarted the current WWE Universal Champion on SmackDown. Reigns will now have to face Kevin Owens at Royal Rumble instead, but this doesn't mean that he will happily co-exist with Pearce from this point onwards.
WWE may need to directly address the conflict between Reigns and Pearce at some point down the line. It doesn't have to be a major event, but Reigns vs. Pearce could still be a likely possibility on a future episode of SmackDown.