Personally I think Fable 4 would be a good time to go into the old kingdom. I've already imagined a storyline myself that, to me at least, is completely canon; the player is a forgotten hero of the old kingdom (the one destroyed by the spire that has been referenced since Fable 1). Heroes in this game are a little more common (in Fable 1 none ever seemed to venture out of the guild, except for main characters who just stood around in set locations), and are vastly hated throughout the kingdom; remember, the spire was destroyed the old kingdom in a wish to make a new world, because the old one was corrupt. Other heroes would often be evil, and would attack the player, though as the player becomes more famous they would learn to fear him/her. To get around the problem of having to go back to just bows, the player would be friends with an inventor; in Fable 2 no one ever really says where guns come from, so I'm thinking that perhaps this inventor is the one who first made guns, and that his blueprints are among the ruins of the old kingdom, to be picked up by an oppertunist before the events of Fable 2.
For the storyline, I haven't really thought much on it, but basically the player was raised by another hero. This hero was evil, to escape the growing trend of the player constantly being raised by good people, and at some point the player would have to be given a very difficult choice; kill their own father/mother for their crimes (or perhaps, if the player is evil, so that they can be number one and not a sidekick), or let them live and run? I'm thinking that perhaps the player is betrayed by this parent in some sort of crime.
Now, for my big totally canon 'omg' game ending that in my opinion would be awesome to see; throughout the game, the spire is being constructed, and as the game nears an end and it's finished, the hero has to get up to the top and stop the game's antagonist from making their wish. After all of this, the player gets to make their own wish. Now, in Fable 2, they never said that there were a specific set of wishes; they just gave you the choices. So, the player would be given a rather more broad range of wishes than in Fable 2;
- Wish for a new world. This would be the canon ending, with a cinematic that shows the world under a new name; Albion. There would be some sort of reference to the Hero of Oakvale at the end of the cinematic, and at the beginning it would show the old world being swept away. I've got a couple of ways around the issue of continued gameplay; either the player is said to have given him/herself a year or so to make the kingdom right (infinite time for the player of course, and as shown in the storyline of Fable 2, the player failed to make the kingdom a good place to live, but succeeded in making a new one.) The second idea is that the player was sent to his own world, identicle to the one he left but still not real, where he would dwell for eternity. This would open up Fable 5 ideas; perhaps the player in Fable 5 found a way into Fable 4 player's Albion? This would also allow for an epic co-op game; the second player would be able to choose files from Fable 4 as well as Fable 5, meaning that they would have an entirely different set of clothing, weapons, even spells, and this would greatly improve diversity. It'd also improve sales I think, since the game would be much more worth playing if you could import a Fable 4 character. That kind if difference would melt my eyes with its awesomeness, though of course the game would have to assume that the player chose to make a new world even if they didn't. If, by the way, the F4 player is evil, then it can be explained that the player simply wanted all the people who ever so much as looked at him/her wrong to die by their hand, and that was the purpose of their wish.
- Wish for money. This one explains itself really.
There'd be other wishes, but I went to bed at 1am last night and I'm not thinking very well
Sorry I waffled on so much there by the way, I'm not particularly expecting anyone to read much of it, but personally I think it's a decent idea. Any thought, to anyone who may have actually read it?