Is Starbuck a Cylon?
Then at end of the last episode of Season 3, when a huge Cylon force has the fleet pinned down, Starbuck shows up in her Viper, saying she's been to Earth and can lead the fleet there.
Warning - there be
According to Starbuck, she blacked out in the gas giant, woke up above Earth, took pictures, blacked out and showed back up at the fleet. According to her ship clock it has been 6 hours. But the ship is brand new and has no nav data.
The logical explanation: she's a Cylon and this is a Cylon trick. We also know there is one Cylon model left to be revealed. Some Cylons have had a fascination with Starbuck, and there's that whole her drawing mystic symbols that match stuff on the way to Earth.
Since that is the obvious answer, it has to be wrong. Unless of course the writers expect us to think that way, so it's a double cross. But then if we know that they'd know... wait a minute, where was I?
If Starbuck is not the elusive Fifth, someone else has to be, and ther has to be some sort of explanation for what has happened with her since her Viper went boom. (Please, Gods of Kobol, not the White Light Bastards from the original series.)
Who else is the Fifth?
Bill Adama? No. He's had plenty of kids and that would also just be too sacrilegious even for these writers.
Lee Adama? No. He's Bill's kid and while Cylons have a tendency to be bat-shit crazy, I don't think that they are programmed to be wishy-washy.
Gaius Baltar? No. That would wash away his whole bat-shit craziness and they wouldn't have needed Six to seduce him to begin with.
Laura Roselyn? Strong maybe. It would be ironic, she's currently leading them the wrong way, and she's had the same visions as the Six in the brig.
My left field pick... Tom Jareck. He's a rebel, he's fallen off center stage so it would be a surprise and just how ironic would it be to have Richard Hatch play the last Cylon?