#4 Finn Balor
Finn Balor initially signed for WWE in 2014 and wrestled in NXT where he had some memorable feuds with Samoa Joe and Kevin Owens, and his title reign of 292 days as NXT Champion is still the longest in history. He made his main roster debut in July 2016 after being signed by Raw, with Stephanie Mcmahon picking him only after Seth Rollins in the WWE Draft.
WWE had high expectations from the Demon King, who became the inaugural Universal Champion after defeating Seth Rollins at Summerslam, his debut pay-per-view. But unluckily, he had to relinquish the title a day later after suffering a shoulder injury in that match which ruled him out for 6 months.
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Since his comeback, he has been involved in decent storylines against Bray Wyatt and Samoa Joe but his match against AJ Styles at TLC showed his true credentials. He won that match but lost to Kane a day later and doesn't seem to be in the title picture.
Balor has never competed at the Showcase of the Immortals, and a match against John Cena could just help him kickstart his career. A storyline where a heel turn for Balor could surprise fans and help him return to the main event picture.