PWInsider.com is reporting that WWE is considering The Undertaker vs John Cena at WrestleMania 26 because the company feels it needs to do something big and unique with The Undertaker at this year's event. Obviously, The Dead Man doesn't have many WrestleMania's left in him, so WWE wants to make sure each Mania appearance is special.
Another rumored main event for the big PPV next year is Triple H vs Shawn Michaels. Although the two have wrestled against each other in the past, they have never done it at WrestleMania, so there has been discussion of booking that bout for the show.
At one point, WWE was planning on booking Rey Mysterio vs CM Punk for the show, but considering Mysterio's current injury, it might not be possible.