4: Bobby Roode
Roode was one of the best heels of NXT but on Smackdown Live he has worked as a babyface since his debut. If WWE really wants to use Roode to his full potential, they might need to consider turning him heel once again.
The Glorious One is veteran when it comes to wrestling and can pull off a clutch performance. Throughout his career, he was a wicked heel who outsmarted his opponents with heel tactics. AJ Styles' agility against Roode's methodical moveset would create magic inside the squared circle. Also, Roode's Glorious theme song in a WrestleMania atmosphere could be a moment worthy of many highlight reels.
Both men have a long history from their TNA days so they know how to bring the house down with each other. The question is if Vince McMahon allow two of TNA's top guys to headline The Showcase of the Immortals.