Kanes first encounter with the Undertaker was his first real challenge at Wrestlemania that you thought he would legitimately lose. Other than that, Randy Orton was still doing his whole "Legend Killer" gimmick and the way they carried that feud and the way that Undertaker sold that RKO leading up to Mania, really made you believe that Orton could be the man to end the streak. He was super dangerous. And finally, Edge was a super threat, same uber heel reasons as Orton, but I really didn't think Edge was going to win. The ONLY reason I felt he stood a chance in this scenario is because he was ALSO billed as Undefeated at Mania and I could see that Edge would be pushed as WWE's New Streak for Wrestlemania picking up where Undertaker left off.
At this point in time, Cena is the ONLY guy left who hasn't fought Taker at Mania I can see being a legit threat to the streak. Jericho would be a nice program and a nice feud, but you can't honestly tell me you'd think Jericho would stand a snowballs chance in hell. CM Punk, again, would be AMAZING for me being a huge CM Punk mark, but even if I think with a clear head I know that Undertaker proved in the fall that he refuses to put CM Punk over. So that wouldnt' happen either.
IF Undertaker keeps up his Mania appearances from now until next year, with the proper build up I can see anyone of the young bucks of Ted Dibiase, Cody Rhodes, Jack Swagger (like I said, proper pushes, don't freak out on me. lol), Drew McIntyre, Sheamus, John Morrison, The Miz, Kofi Kingston or hell, MVP could all be viable Wrestlemania challengers.
But that all depends on what WWE decides to do between now and 2011 with their current talent pool. As for this year? If HBK Challenges Taker again he's not winning. John Cena is the ONLY option this year that I can reasonably believe that Takers gonna give the streak to.