#4 Kane
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Like The Undertaker, Kane has also had an extensive career in the WWE, having been with the company for over two and a half decades. But, with him now the mayor of Knox County, and him also being over 50 years of age, having him back in the ring for a match seems like it wouldn't make sense.
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Kane has achieved everything there is to achieve in the WWE and perhaps doesn't have anything left to offer inside the ring.
And with his mayoral duties keeping him busy, I don't think he will be available for a lengthy feud as well. I think Kane should also go into the Hall of Fame, much like The Undertaker, and hang up his wrestling boots.