Samoa Joe
This one is more of a personal preference, but Samoa Joe seems like the kind of superstar who could feud with anyone.
He's so talented for a man of his size and he's great on the mic. Any potential match would have seen Joe carry the action.
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It would have been easy to book because Joe has constantly shown that he isn't afraid of anyone from Brock Lesnar to Seth Rollins to Roman Reigns.
Joe has been under-booked for most of his main-roster run as he's usually been a main-event foil for the likes of AJ Styles, Reigns and Brock Lesnar.
But since he's so athletic anyways, it would have been fun to see Joe take on someone so much bigger than he is.
The whole 'disappear and reappear once the lights go out' would likely have spooked Joe, but had he been signed earlier by WWE than he could have easily faced the Undertaker at a PPV.