#3 The Undertaker

WWE attempted and mildly pulled off a match between John Cena and the Undertaker at last year's WrestleMania. It mainly consisted of Cena acting like a fan until he 'received word' that the Undertaker was in the building.
The match eventually happened but served as more of a squash match in favor of the Undertaker.
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If Cena wanted to get his win back or have a proper match, it wouldn't be hard to conceive another storyline between the two culminating in a better match at WrestleMania 35.
The Undertaker isn't going to be suiting up for many more matches, but he was able to make it work for two matches with Triple H late last year. He appeared at both Super Show-Down in Australia, and Crown Jewel in Saudi Arabia.
Since he still might be able to put on a believable match, another showdown between the two future hall-of-famers isn't out of the question.
Cena and the Phenom aren't going to be able to have a big match against each other for many more years, so the WWE might strike while the iron is still mildly warm this April.
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