As for something HBO could do after GoT, I think it would lose a lot of strength by going back over the previous 0-40 years. A lot of the strength in the idea of having made the show start with that is all the show-only watchers wouldn't know anything about the book stories, so the Robert vs Rhaegar, Jaime kingslaying, and the loads of other awesome stuff would not have been known beforehand (except to those who read the books, but fuck em amirong?). What I think they should do is make a second series about Old Valyria, pre-dragon, pre-Targaryen, pre-doom. It would basically just be creating whole new storylines and new lore with a solid foundation. It would be nothing other than GoT part 2, but made specifically for TV, which would make the plotting much tighter IMO