AJ Styles
Like Samoa Joe, AJ Styles could have easily squared off with the Deadman had WWE decided to sign him a decade ago.
But due to their previous bias against superstars from the former TNA, Styles only joined the WWE in 2016.
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The Undertaker was still technically active for WrestleMania matches, but WWE wanted some of his final matches to be against John Cena, Reigns and the likes of DX.
Styles could have easily carried the Undertaker to a great match as he's done so against the likes of Brock Lesnar and Samoa Joe over the years. While it wasn't a necessarily great match, he also carried the action against Shane McMahon.
He's a big-enough name in the business and it would have made sense to have Styles challenge or be challenged by the Undertaker at some point.
I'd rather have seen Styles face the Undertaker instead of someone like Cena, Reigns, Shane McMahon or Brock Lesnar, but he wasn't considered a big enough star when most of those matches went down.