I actually think that the Undertaker's final Wrestlemania match should be against John Cena.
I saw someone earlier compare such a match to Hogan/Savage from Wrestlemania V, but actually I think a better comparison would be Andre the Giant vs. Hulk Hogan from Wrestlemania III.
In 1987 when they fought Andre was the Eighth Wonder of the World, billed as being undefeated for 15 years, and was that generation's version of the iconic phenom. He was also the leader of the locker room and idolized and respected more than anyone else on the roster. And Andre was in the final stages of his career, the twilight of his career.
25 years later, the Undertaker is in the same position. The Undertaker is now the legendary phenom, leader of the locker room backstage, more respected and idolized than anyone active on the roster, he's also undefeated at Wrestlemania. His undefeated streak has as much mystique and hype as Andre's billed "undefeated streak" in the WWE. And like Andre was in 1987, Taker is now towards the end of his run after over 20 years in the WWE (which is even longer than Andre was in his WWE tenure in 1987), the twilight of his career.
Hulk Hogan was the superstar, pop culture phenomeon who was the top babyface and the center of the company. The biggest draw, the biggest hero to the kids. The All-Star. The Franchise.
25 years later, John Cena is in a simliar boat. While he's not as big a pop culture icon as Hogan was in the '80s, nor is he as all around popular with the entire WWE audience as Hogan was in the '80s, still Cena is the all-star, franchise of the company, and the leader of this era.
Andre was the biggest attraction of the previous era before Hogan, just as Taker has been in the era before Cena.
If Vince hypes this right, Cena/Taker for this era could be as big as Hogan/Andre, only with a much different twist.
The biggest difference between the two is that the roles would be reversed, largely due to one thing, smarks. Whereas the audience was ready for Andre to pass the torch to Hogan, the audience will not be near as receptive to Taker passing the torch to Cena. Kids will, but teenagers, and young adults and the adults that grew up towards the end of the Hogan era and into the New Generation era and the Attitude Era will not want Taker, or most of them anyway, to end the streak by losing to Cena.
But even though it wouldn't be near as much of a clear cut evil/good battle as Andre/Hogan was, it would still be just as significant and epic. Especially if the match is billed either flat out or subtlely as Taker's final match. If that happens, with Cena being seen as becoming the official leader of the company it will seem obvious that Cena will win and the Streak will end. But will the streak really end? Thus this could be the most unpredictible WM match of Taker's career, which is perfect.
As far as will Cena need to turn heel for it? Absolutely not. Keep this dynamic. Keep Cena as the face for the kids, while still getting booed by your smarks and old school fans that don't want Taker to lose to Cena. Who would win, you could go either way. Instead of an good/evil dynamic that Hogan/Andre had, you'll have more of a babyface epic encounter ala Hogan/Warrior, Hart/HBK, or Rock/Austin, but with the significance of Hogan/Andre, aka: quite possibly the biggest matchup ever.
There's one problem though that will make this match very different from the Hogan/Andre match. Cena cannot beat Taker and become the beloved hero for doing so and ending the streak. If it was 1987, absolutely. But it's 2011. But if you want to turn Cena heel, this is how you do it.
Have him cheat to beat Taker and end the streak. Cena doesn't need to be put over clean by Taker. Cena is already the top dog and star. Taker will basically pass the torch by losing no matter how he loses. Cena could be the biggest heel ever if he cheats to beat Taker and end the streak. I can see trash being thrown in the ring and absolute digust and hatred from the fans with Cena celebrating and gloating after winning. It would be priceless. And you'll definitely have the biggest heel turn since Hogan's turn at Bash at the Beach 96.
So yeah don't turn Cena heel leading up into the match. If you want him to be heel, have him end the streak. Him winning alone will turn him into the biggest heel perhaps ever. And it will be gold.
However, that doesn't mean that's what I want to happen. I want Taker to retire with streak intact. I think he deserves it after the amazing career he's had and how much he's given to WWE and the fans. But if the streak has to end, this is the way to do it.