Joseph Adama as a Cylon
I previously wrote a bit about slavery and have since learned that indeed, those developing the Caprica prequel intended primarily to cover that topic in the series.
So let me advance an interesting, if only modestly probable theory: Joseph Adama is a Cylon.
As I noted, it now seems likely that Joseph Adama protested the development of the Cylons because they would be slaves. This would make even more sense if he was our mystery #1 Cylon. The show, we are told, tells the story of an often repeated cycle. At least three times, it appears, humans have created Cylons (presumably as slaves each time) and there has been a war, and the "humans" have fled into space. It is not clear to me if the Final Five, who have watched this several times by now, would like to break that cycle or feel they must guide it. However, it is interesting to imagine Joseph Adama as a Cylon trying to break the cycle, trying to convince the colonials not to make a new race of slaves, who he knows will eventually revolt and destroy them.
He fails of course. But it does explain how his associates have gravitated around his son and grandson, and how they escape the genocide and lead the fleet away on the 3rd (at least) exodus.
The main reason I find this interesting is that the revalation would be dramatic. Even though Joseph Adama is an unseen character on the show, his arrival as a Cylon would create a great dramatic reveal. And of course, the reaction on the faces of the younger Adamas. Plus, as we know, the sometimes prophetic, sometimes lying Leoben declared "Adama is a Cylon."
Update: New information released about the prequel Caprica series adds a lot of complexity to Joesph Adama, the Adama family and the Cylons. They can be read as making it far less likely, or oddly, more likely, that he could be the final Cylon.