#4 Seth Rollins
While they have clashed in 6-man tag team action, The Undertaker and Seth Rollins have never faced each other in a singles match. But this story writes itself. Seeing the Monday Night Messiah take on the Unholy Trinity of the Deadman, American Badass and Mark Calaway would be a sight to behold.
Rollins would be one of the few Superstars who can guarantee a great match for The Undertaker at this stage, with his immense selling ability. His villainous traits could carve out a story as personal as Taker's feud with AJ Styles.
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It could all begin with Rollins wanting to sacrifice The Undertaker's spirit for the "greater good", with the potential for a variety of matches. The two could have a wildly entertaining Boneyard Match, if there will ever be another one, with Murphy and Austin Theory trying to come to their leader's aid.
Another option, if Undertaker has one more match, could be a traditional Survivor Series match between a team of his choosing and Seth Rollins' faction. Having his last WWE match be the same one he debuted in 30 years ago would not be a bad way to say goodbye to The Undertaker.