So the Cylon Civil War is followed by a Colonial Rebellion. I'm guessing now that it's been "Earth or Bust" for so long now and it's turned out to be "Bust" anyway, Zarek decides to seize power once and for all, or maybe he's pressed into it by the Quorum.
Well, we can scratch Billy off the list of possible Final Cylon suspects now and maybe Gaeta. It's not a guest star either, so that limits the field by eliminating Ellen and Romo Lampkin. That seems to narrow it down to Dualla, Boomer, Gaeta, and Cottle or a duplicate of another, regular Cylon or human character, which, speaking for myself, seems more unlikely than Boomer being the Final Cylon.
Ground battle, eh...I'll bet it's either with the insurgent Colonials or more likely the other Cylons show up to take the planet. After all, why would the humans want it? The Cylons could possibly reclaim it. My guess here is that the rebel Cylons decide to ally themselves with humanity for the sake of everybody's survival and the other Cylons aren't too happy about it. The Cylons could reclaim Earth so humanity could live on it, which sounds like that's the "higher power's" intention anyway. I'll further guess that Cavil's Cylons turn up at some point in the Colonial rebellion and the humans swiftly have to come together to fight them off, and they determine that they need the rebel Cylons in order to survive.