Two words: time travel.
There's just no other reason to introduce a black hole/naked singularity/what have you in a science fiction series at this point. And no reason to wait so long to introduce its existence, either. If they'd been saying "all of this has happened before, and all of this will happen again" alongside the constant presence of a black hole all season, we'd have seen this coming a mile away, but without the black hole it's easy to take the phrase as being more metaphorical.
The basic idea is that, at some point while battling The Colony, they jump into the black hole/singularity, and go back in time. Possibly to Earth...but probably to Kobol. On Kobol, they find themselves reenacting the events in the Book of Pythia; they become the Gods of their own legends, and start the cycle anew. For example; Athena is named after one of the Lords of Kobol. The Athena of old killed herself in despair by jumping from twin mountains named the "Gates of Hera." I say she reenacts this in some form after losing Hera in the Opera House.
The idea that they become their own ancestors explains a lot. Most prominently, it explains why there's a "Temple of Five" that seems to predate the Final Five. That aspect of the timeline here has never made sense, logistically, but this makes sense of it.
It also explains how the 13th Tribe could have been made up of Cylons. Perhaps they all settle on Kobol, but not in perfect harmony; we still have Cylons like Tory who believe in the idea of a "pure" Cylon-only society. So, they keep to themselves a bit, break off from the rest of the "mixed" tribes, and next thing you know they're heading to Earth (which, in the past, hasn't been nuked yet), and round and round we go.
It also ties the show's themes of techology and mysticism together: the Lords of Kobol were real people, but they weren't actually Gods. The myth has been idealized and morphed over time, but was based in real events, which they themselves are about to engage in.
There are lots of smaller details that make a lot more sense when viewed in this light, but this covers the big stuff. I realize this show has made fools of all of us, but the more I think about this, the more elegant it seems. I'm convinced that this has to be it.
Another reason this fits...
...Ron Moore, IE: the BSG showrunner guy...wrote a number of episodes for Star Trek: The Next Generation. Among his credits is the series finale, in which Captain Picard (spoilers within spoilers coming up!)...solves a problem in three different periods of time simultaneously. It all turns out to have been a test for humanity administered by a near-ominpotent race of beings (the Q Continuum), which they pass.
So, not only does this make sense of a lot of the dangling plot threads, but this wouldn't even be the first time Moore has used time travel in a sci-fi series finale.