If the final Cylon is not Baltar or Roslin, we are being cheated. [Updated comments 7.28.08] And it looks like we will be. None of the lesser characters not pictured in the so-called Last Supper portrait released for PR purposes prior to Season 4's broadcast debut will suffice. Gaeta or Dualla are most likely, but It will be a massive letdown if it isn't Roslin or Baltar. Others in the blogosphere have pointed out that the Caprica spinoff has Dualla's family being tied to the Adamas. At the ComicCon in San Diego [7.27.08] David Eick joked that when picking the Final Four they went back-and-forth between Gaeta and Anders and chose Anders, so Gaeta may still end up the Final Cylon.
The writers have foreshadowed Roslin twice (she felt something in her head prior to Cylons doing something - once at the brig and at the end of Season 3 in Galactica's C&C). Baltar makes a much more interesting Final Cylon - think Doctor Daystrom and the M-5 in the original Star Trek.
Obviously civil war breaks out among the humans and among the Cylons, with a melding of some of the Cylons with some of the humans. Most likely the Brother Cavils will emerge as the main bad guys who still seek the genocide of humans. [Predicted correctly]
D'Anna will be the one to lead them to Earth, not Kara. D'Anna is now the enlightened one and she will lead the faction of Cylons that allies with the humans who survive. [D'Anna played a part "The Three gives you the Five," according to the Hybrid] It is the Cylon Centurions, sent by the female Hybrid, who awaken her. [It was actually a Cavil who brought her back]
There will be a great final battle - probably at the asteroid belt between Mars and Jupiter. [It is coming - sounds like in the last few eps - but where is YTD]All of the metal models will die there and all of the Cylon ships will be destroyed or die there. Several of the Cylon models will die and most of the humans in the Colonial Fleet will perish as well. The asteroid belt is significant because this is the place the virus which kills the Cylons resides - a natural barrier to the Cylons ever reaching Earth. The handful of Cylons who do make it to Earth must join with humans in that special way to gain the antibodies needed for protection against the virus. [YTD]
Adama finally steps forward as the active hero - taking upon himself all of the leadership responsibilities as Roslin fades. [YTD and very much in doubt now that they gave him a nervous breakdown]
The CYLON PLAN: To break the cycles of time. They feel trapped. The winning side in the Cylon civil war believes they must do anything to break the cycle, which includes allying with the right humans in the right combination so that time can be free from the feedback loop in which humans and Cylons are trapped. [YTD - it looks for now that they are going to simply ignore "the Plan"]
Kara's Vortex: A small black hole or wormhole perhaps (even though science has now discredited wormholes as a possibility [ref: Stephen Hawking]) - which made her travel possible from the atmospheric "deck" of that vortex planet. The flashbacks (Mother)? Hmmm. Don't know what they're doing with that, and, frankly, don't care much. Starbuck should have dreams all right: of another cycle of time – she can see a different past or future (not sure which) in which she is with Loeben as his mate. [YTD - instead it sound from ComicCon spoilers of the 4.5 trailer that Kara and Lee reunite]
Baltar becomes a true villian - in the full grip of his megalomania - and James Callis gets to chew some scenery again! [Predicted correctly - at least partially]
Origin of the Cylons is revealed: We'll see a flashback of an AI (artificial intelligence) program becoming sentient and having machines build other machines. [YTD]
Roslin becomes a zealot, convinced that she must immerse herself into Scripture and Kamala, and gain insight to find the path to Earth - we'll get more cool dream sequences (are there ever enough of those?). [Partially correct - so far ... and by D'Anna keeping Roslin close, perhaps Roslin is the Fifth after all]
The only correct way to end the story is around 1500 B.C. in the eastern Mediterranean - Ron Moore and David Eick must resolve why the Earth Greek God structure is meaningful to the Colonies. Research Minoan culture and the eruption of Santorini and you'll begin to get the picture ... for example: Greek mythology immortalized Crete and Knossos with its legends. According to the Greeks, Mount Ida (on Crete) was the location where Rhea, the Earth Mother [Hybrid?], gave birth to Zeus [Baltar?]. He was fed by nature a diet of honey and goat’s milk, was tended by a group of nymphs[see Episode 1, Season 4], and was guarded by an army of youths against his father, Cronus [Adama?], whose reign was threatened by Zeus’ existence. Zeus fathered a son, Minos, who became the King of Knossos, Crete, and the rest of the Aegean. And that is just one possibility. My favorite scenario is a thriving Atlantis - the island of Santorini before it blew up - figuring prominently in the ending. Various of the surviving characters would be perceived as godlike by the locals and thus could give rise to the Greek gods structure and legends.