Who would he challenge at WrestleMania 35?
Daniel Bryan lost out on this one by virtue of a coin toss.
Daniel Bryan and AJ Styles are two of the greatest wrestlers of all time. They can have excellent matches with anybody. They are also both massive stars in their own right.
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At this stage of his career, The Undertaker's opponents need to have two features.
The first is blockbuster star-power. Because he is such a big star, The Undertaker can overshadow his opponent completely if they do not have sufficient star-power. Look at the case of Rusev and Aiden English at the Royal Rumble.
The second, arguably less important feature, is wrestling ability. The Undertaker isn't going to have technical classics at this point of his career. He isn't going to carry anybody to great matches either. His body is too broken down. However, he is still The Undertaker. He still has the aura. His psychology remains as good as ever. All he needs is a ring general.
AJ Styles fills both those criteria. He is one of the WWE's biggest stars, and one of the greatest wrestlers on earth.
The Phenom versus The Phenomenal One. For the WWE Championship. At WrestleMania.
The Verdict: If he wants to have one last tilt at a world championship, he will win the match.