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. ...