Well lets look at recent history
Wrestlemania XX: Pretty much an orgy of pretty big name stars. Harley Race may have been considered the front runner, but Jesse Ventura, Slaughter and others were just as big in WWE terms.
Wrestlemania 21: A pretty large orgy of stars as well, but the star of the Show was Hulk Hogan. Roddy Piper is as big in some circles as Hogan, maybe not realistic, but not beyond the realm of possibility. Hogan was the Star, but Piper had a pretty good second billign.
Wrestlemania 22: The star obviously was Bret Hart. It was Bret Harts' swan song. Eddie Guerrero, rightfully or wrongfully, had a pretty good send off as well.
Wrestlemania 23: Probably the most lackluster of the recent Hall of Fames, but it was a night to remember the American Dream, Dusty Rhodes. While not significant by any stretch of the imagination in terms of WWE success, his NWA career was rock solid. Second billing of the night went to Curt Hennig. Was Mr. Perfect a big enough star to go number 2, who knows.
Wrestlemania 24: As much as the night belonged to Ric Flair, and is was his show, the Rock would of have you believe that Rocky Johnson actually meant something to the business instead of simply being the guy with the right skin tone at the right time. It was Ric Flairs show, no second billing, and if there was, it was because a guy was giving a speech, the person being inducted was inconsequential.
Wrestlemania 25. Without question it was and si going to be Steve Austins night. It'll be interesting to see in the next few weeks who will get second billing. If history indicates anything, it'll probably have a semilocal appeal. I would imagine that the Von Erichs and the Funk will fit that bill.
So to go with Wrestlemania 26, we'll look at a few things. The WWE has no problem with putting an NWA guy at the top of the billings. The WWE has headliend 3 with Hogan, Hart, and now Austin, with the NWA getting Rhodes and Flair, and arguably Harley Race if you go that route.
Also number 2, you're thinking about location. The main star usually has a somehwat local flavor to it.
So who gets the nod. Not the Rock. The Rock I think is on thin ice with the WWE after the stunnt he pulled at last years HoF. The Rock selfishly made the night about his family, and tryied to steal the show, so he's a no go. A twin billing of Randy Savage and the Ultimate Warrior would be pretty awesoem in my opinion, but very unlikely.
I'm going with a wild card, Sting. If this is indeed Stings last year in TNA, and last year in the business, he's going to eventually take a WWE legends deal that doesn't require him to work a single date. Guys like Flair are going to talk into h is ear. The possibility of a final big pay day, with a DVD that commemorates his career, I think it will be too much for the Stinger to pass up.