Roslin may well be the final Cylon. Or perhaps the "final Cylon" is not a Cylon at all - but of the original guardian stock - the "first cylons". We'll see. My money is on Gaeta as the final cylon, who will take steps at the last to stop Baltar - who will have the delusion that he is the final Cylon and is required by that deluded status to do something *stupid*.
FWIW, I think the ultimate artificial humans in BSG are going to be the caretakers of Earth.
If "all of this has happened before and will happen again" mantra is correct, then I think this series will end in a nod to Asimov. Whereas Asimov's robots looked after an irradiated Earth, RDM's robots will have been the guardians of an Earth destroyed by an atmospheric & ecological catastrophe.
I think that RDM' will write it so the Earth was the birthplace of the Colonies (humanity). Earth was abandoned for ecological reasons and its guardians, artificial humans, returned to repair it to await the return of Earth's children. They were able to survive on the now harsh Earth while humans were not. The original "cylons" left Kobol for Earth, while the humans left to form the colonies.
These guardians of Earth are far more technologically advanced than either the Colonies or the Cylons. While the Colonies and Cylons are warring with missiles, the Guardians will be armed with beam weapons.
I entirely look forward to the Colonial Fleet reaching Earth, finding it more or less pristine with its environmental damage now repaired, only to have the Cylon fleet jump in and launch an attack to utterly destroy the fleet...
...only to have the attack get interrupted as the android protectors of the Earth launch an attack from the Moon wielding Star Trek style beam weapons which wipe out the Cylon fighter wave in a breathtaking orgy of superior technology in about 40 seconds. **BOOM**
*silence*
Then the hailing frequencies will open and all will be revealed. It would be pretty shocking to have Roslin die four or five episodes from series end from cancer - only to have her revealed in that moment as the leader of the guardian Cylons now protecting Earth.
Anyways - that's my uninformed and pure speculation bet on the series end.
I just hope we get to see a series end - some ending. It would be a great disappointment if NBC pulls the plug on the series now as it has been rumored may happen.