Yeah, I mentioned this in another thread, but there are four guys I want to see fight Undertaker at Wrestlemania before he retires: Jericho, Swagger, Sheamus, and the guy to end his streak, John Cena. Swagger probably makes the most sense for next year since they're on the same brand, but Jericho or Sheamus would work as well.
To answer the thread question, do I see Sheamus headling Wrestlemania next year? No. WWE may push guys straight to the main event, maybe even give them a world title early, but the main event at Wrestlemanis is usually reserved for guys that have been there awhile. And when it is a relatively fresh face, it's out of necessity because they've lost a lot of main eventers, like when Batista stepped up after Rock, Austin, Lesnar, and Goldberg had all left.