@Wrestlinfan608: What The Brown One; and Adreme said is true. The person Undertaker faces at WM 28 has to be someone who we actually believe can beat him at WM. It's more a show of respect than anything else. There are only a few wrestlers on the roster now that could establish that believable moment in history while at the same time honoring and showing respect to Taker and his accomplishments. Here's my list of who I think possibly deserves that most honored of spots at WM, and reasons why. (Not listed in any specific order):
Triple H - Undertaker and HHH have a history a mile long. Their first WM meeting was at WrestleMania X-Seven, then again at WrestleMania XXVII. The story build up revolves around Shawn Michaels being friends with HHH, and also with the respect for Undertaker The Game talked about in the build up for last years WM. Not to mention the beating he gave Taker last year and the remarks at the RAW afterwords stating that he would be waiting for Taker to return.
The Rock - He has never faced Taker at a WM, however him and Taker do have history with each other in their feuds over the WWE Championship during the Attitude Era. Rock has all the credentials to make it believable that he could possibly beat the Undertaker at WM, however he is already booked in the main event vs Cena.
Kane - Everybody who has watched wrestling knows the history here. The Big Red Machine is a very believable match for the Undertaker at WM. And there's not a lot to say about this match up, it has it all. Yes it's been done before, but it wasn't with so much riding on the line.
John Cena - Superman can do it!!! Seriously though, Cena has the experience and respect for his peers so as to make this match up a huge selling point at WM. It is believable that he could win with his 5 moves of Doom considering Takers current state. However he is also already in the main event at WM vs the Rock.
Randy Orton - He pushed Taker to his limit at WrestleMania 21. He's crafty, has experience respect, and a history with Taker also. What a feather in the cap it would be for Orton to cement his legacy as "The Legend Killer" by ending the streak at WM 28. It's believable, and could have a great build to it.
Christian - Captain Charisma himself. Why not? He has experience, he respects his peers, he crafty enough to pull it off and his "one more match" fits in with Takers "one more WM".
There are a few more but it's getting late for me and I have to get to bed. Anyway if Taker wants a match at WM, it really should be against someone that has the whole package, both inside and outside of the ring and backstage. This is of course my opinion.