According to Edward James Olmos, we WILL find out what happened to William Adama after the events of the Battlestar Galactica series finale, because he's at work on a sequel we'll all get to see "pretty soon."
While discussing his film Selena during Hispanic Heritage Month, the conversation (as you might expect) eventually came around to BSG, and Olmos said:
"I'm going to go there. I'm going to go there in a graphic novel. Pretty soon I hope. I don't know. It just depends on whether people can get behind it and understand it for what it is. I think people will. ...
"I want to know what he's doing right now, don't you? I kind of was left with him sitting about to build his cabin, so God almighty I'd like to know what's happening to him right now. In many moments throughout the day, I'll think, 'I wonder what he's doing? What the heck is he up to? What the heck are those people up to?' Here they were 200,000 years ago on this earth. They came from the experience that they came from. All I can tell you is: aren't you guys curious as to what happened to them?"
Even though Olmos promised his graphic novel take on BSG "pretty soon," it may take a while yet, because the actor is still in the process of finding the artist to collaborate with, but whenever he pulls this project off, we're there. How about you?