#2 AJ Styles
Prior to WWE deciding to go back to the brand split, AJ Styles had an incredible WWE World Heavyweight Championship program with Roman Reigns. In fact, you could make the argument that Styles has been Reigns' best opponent to date.
It's been a few years since Reigns and Styles have been in the ring with each other, and on top of that, "The Phenomenal One" has been on SmackDown Live for the majority of his WWE run, so it's time to have him switch brands.
It's likely that Styles' last major match as a part of SmackDown Live will be against Randy Orton at WrestleMania 35, and after he presumably beats the "Viper," he'll move on over to Monday Night Raw and feud with Roman Reigns, as the two have incredible chemistry in the ring.
Reigns might end up being booed in the feud, however, the match quality would be top notch, so it'd be worth it.