The 5th Cylon is ShaBoomTha

So wait, there's Caprica Six, Baltar's Inner Six, Shelly Godfrey, Gina Inviere, Natalie and even more than that? I don't know about you, but with all the multiple models running around, it gets a bit confusing. And not only does Six have multiple models, but so does number Eight (aka, Sharon, Boomer and/or Athena, or ShaBoomTha). Who knows, maybe there's a twin Cylon running around out there. We ARE waiting on number Seven, who's to say that ShaBoomTha doesn't have a twin sister (totally NOT a brother) running around out there?

Why it works:

Twin Cylons! What a concept! And we've been kinda lax on the ShaBoomTha storylines as of late, so let's thrust her, AND her twin, back into the mix. First off, it would just be wacky (think Parent Trap with a Colonial Fleet flavor). But it really lends itself to some great episode arcs -- Helo fraks the wrong one, Hera doesn't know which one is her mom, and then the whole doppleganger two-parter would rule.

The twin angle really works because there wouldn't have to be some huge back story. Nor do they have to increase an actor's pay, she's already playing multiple parts as it is. Maybe the twin parts her hair the other way, or is left-handed instead of right. She could even just be a mirror opposite of ShaBoomTha, so her name would be AhtMoobAhs (similar to the Cobra Twins).

Why it doesn’t work:
Would the writers truly throw that idea at us? Maybe. Would it work? Definitely not. While it's odd that there are multiple versions of each model, a twin Cylon would be the lamest thing ever.
The last thing that BSG fanboys want is another Gollum-like story only to find out that there really are two Gollums.

Hmmm. On second thought, fanboys might like that.

Odds that I’m right:
662:1