I really don't think Orton is ready to main event a WrestleMania one-on-one. Mr. Kennedy, Edge and King Booker are three heel acts I would put above Orton. Considering WrestleMania is meant to be the biggest show of the year, and there are three World Championship, that means that even if Kennedy, Edge and Booker were occupied elsewhere, Orton still shouldn't get the title shot.
I also don't want to see another Triple Threat at WrestleMania. We've had two recently, and it's just...bleh. Not another Triple Threat. If you added Shawn Michaels to the match? Now we're talking. Orton can counter the F-U into an RKO in that match, then get Speared by Edge for the pin.