That guy in the fedora? That's the Phenom, the Dead Man, the guy who retired Shawn Michaels...looks a little different than usual, right?
His unbeaten streak at Wrestlemania is one of the biggest accomplishments in wrestling history, and taking more than half the year off (so far) is a pretty good sign this Wrestlemania will be Undertaker's last match.
If Taker does have a match at Wrestlemania (and even that is questionable), it will need to be against someone he already has a long time feud with so he won't have to build one on TV. So that crosses off Wade Barrett, Chris Jericho, Alberto Del Rio, CM Punk and Daniel Bryan (someone said Bryan might cash in his briefcase on Taker's streak...)
It also has to be someone who can put on a good match. Taker's last match will not be two giants just slugging each other. This crosses off Big Show, Mark Henry and Kevin Nash.
Others are either in TNA or aren't wrestling period. This crosses off Sting, Samoa Joe, Ric Flair and Stone Cold.
I also wanna dismiss Randy Orton as a challenger. He has not been seen as a legend killer in a while, and being the top guy on Smackdown means he is going to be involved in the WHC match in some way at Wrestlemania.
Some Hope: Kane (exception to the big man rule) and Mick Foley both have a long history with Taker and may still be able to put on a really good match.