#4 Kane
Why not just go back to where it all started? Why not make the final match of The Undertaker's career be against his own brother, Kane? Furthermore, why not use some of the cinematic type storytelling that the company has been utilizing lately to create a match up of supernatural proportions.
Both The Undertaker and Kane have had Hall of Famer careers in WWE. But, both legends are far from their best in the ring. While it would be nice to see them face off inside the squared circle one more time, the limited mobility of both men makes a cinematic type match all the more appealing.
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If nothing else, this might not be any sort of dream match or even the best bout on the card, but it would be a way to tell the history of The Undertaker and Kane from beginning to end. A chance to play up that history in a meaningful way and bring the careers of both to a dramatic and beautiful end.