I don't like the idea of a prequel. We already saw the world how it was when Heroes were abundant (what was that first game... Uhh? Fable! Ya that's it). Also, if you listen VERY closely in the game, you can pick up hints to the future of the Fable games. I heard a villager say, "Wouldn't it be great if there was a rifle that fired like, a hundred rounds, without having to reload it after every shot." Something along those lines at least.
So that's hinting at a machine gun. Now, I know a ton of people on these forums look to the Fable games on a historical level parallel to our world, and that's true, but you also have to look at it from the fantasy perspective. We had the medieval age (First Fable), we had the Renaissance (Fable 2), we had a MAJOR time-skip to the Industrial Age. Which started in the 1800's. And Fable 3 is all about Revolution. So, they kind of combined Revolutionary aspects with the future (that's where the fantasy comes in).
I think the next Fable should be about a Civil War. Just about every country has had one. It also would present an AWESOME opportunity. The rise of the Heroes' Guild. Look at it this way; Albion is destroying itself slowly. Heroes were always there to keep the peace, but no longer. Once more, a Hero has to rise up and gather others who have the Archon's blood running through their veins, with this, they rebuild the Heroes' Guild and try to bring peace to Albion.
I think I just went off topic but hell. I think that's the way it'll go (the Civil War part I mean, first automatic gun ever made, Gatling). So... ya.