Republican National Convention schedule hints at climactic mystery speaker. Who will it be?

Could there be starbursts in our future? The Republican National Convention has a speaker set for Thursday night so exciting we can't even know who it is:

Buried deep in the convention schedule released Monday is a vague reference to a mystery speaker scheduled for the event's final evening. 'To Be Announced' has a prime speaking slot late in the Thursday program. [...]

The only other speakers to follow 'To Be Announced' will be Sen. Marco Rubio of Florida and Mr. Romney himself, suggesting that the unnamed guest may appear during the 10 p.m. hour when the networks all will be broadcasting the convention.

The timing says it's a big-deal speaker, but most big Republican deals will have spoken by then. The secrecy says they think it will make a splash. Who, then? The Wall Street Journal offers up Zell Miller, Nancy Reagan, Ted Nugent, Sarah Palin, Gen. David Petraeus and Chesley Sullenberger as possibilities.

Chesley Sullenberger? I don't think it in any way diminishes Sullenberger's heroism in landing Flight 1549 safely to question whether, more than three years later, he's a big enough name to take up some of the Republican convention's precious network airtime. Also, while Sullenberger is a longtime Republican, it appears that this time around, he and his wife are both Obama donors.

Zell Miller? So 2004. Ted Nugent? Come on. The only reason Nugent is even a little relevant in popular culture today is simply that there are so few Republicans in entertainment. Mitt Romney has gotten every scrap of benefit he's going to get from Nugent, and the guy is now only a potential liability. Gen. David Petraeus? That would be a big departure from his staunch non-partisanship to date, and hella awkward since he's President Obama's appointee as director of the CIA, so it would certainly fulfill the splash requirement.

That leaves Sarah Palin and Nancy Reagan. The secrecy would make sense for each for different reasons: Palin's history of backing out of commitments might leave even the RNC nervous, and Reagan's frailty might make her a question mark until the last minute. But I still think they'd be pumping Reagan ahead of time'that way, if she couldn't make it, it would be in itself a climax, that she'd tried to make it despite her age.

And Palin? On the one hand, you have to question whether she could keep a secret. On the other hand, she's going to be everyone's top guess. If it's not her, they risk disappointing and angering her fan base.

Who else could it be? Who's not on the RNC schedule and popular enough to help give Romney a bump?