#1 Keith Lee could dethrone Sheamus

The most likely candidate to dethrone Sheamus is a fellow powerhouse who claims to respect him - Keith Lee. The Limitless One and the Celtic Warrior expressed mutual respect for each other while gunning for Drew McIntyre's WWE Championship in January.
They also teamed up briefly, and while that didn't necessarily play out very well, the two men still respect each other.
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Unfortunately, fans haven't seen the Limitless Keith Lee in four months. He has been inactive since mid-February. Luckily, Lee has vowed that he will return, which is a relief for many.
When Lee does return, he should start off right where he last left. The Limitless One was supposed to compete in a Triple Threat Match with Riddle and Bobby Lashley for the United States Championship. He pulled out at the last second, and the Original Bro won the title.
Later, reports indicated that Lee was originally supposed to win the championship, but those plans never came to fruition due to his injury.
If Lee was around right now, Sheamus wouldn't be US Champion. Therefore, when the former NXT Champion returns, the Celtic Warrior's days as champion will most definitely be numbered.