4 possible finishes for The Undertaker and John Cena's match at Wrestlemania 

#3 John Cena wins clean via Top Rope Attitude Adjustment

John Cena

As per traditional wrestling, it is expected of a wrestler to go out on his back in his last match. It is a way of passing the torch and making sure someone else gets legitimacy from a retirement. As in the case of Ric Flair and Shawn Michaels, both men lost their final match before retiring from WWE, and there is a high chance The Undertaker will follow suit.

It is highly unlikely that Cena will be able to put down 'Taker with a single attitude adjustment, so a couple of AAs followed by another one from the top rope might be what is needed to do the trick.