I'm sure that Orton has quite a few Wrestlemania main events in his future. The biggest problem facing him right now, is that WWE is trying to invest in newer talent, to better strengthen the depths of the Smackdown roster, and Wrestlemania is great showcase for that. Yes, Orton could be inserted into the World Heavyweight Title picture for WM this year, but it would most likely put him into the role of enhancement talent, and he is simply too good, and too over for that. Working with Kane gives him a clearly defined, well established heel to feud with, and a WM victory that will please his fans, and sell lots of Orton merchandise.
Also, there is just no getting around the fact that Orton is the number two guy to John Cena. It has always been that way, and until Cena either retires or turns heel (probably never happening), he'll never be THAT guy. Yes they are on different brands as of now, but given what I've stated above, along with Cena's grip on the WM main even picture each year, he is in a bit of a vice.
But all that said, Orton is really at the top of his game, and one of the most over talents in the WWE. A heel turn, brand switch, or just perfect feud could put him in a WM main event in a heart beat.