I was hit with this irritating plot bunny late last night, and it has just multiplied until I wrote it down.
Credit obviously goes to whom ever does adopt this, though I would like to be given a link to the published work please. Go wild with it, you don't have to use all of the points I mention but it would be nice to see it.
Harry was left to the cold one winter when he was young by his uncle. Magic wasn't pleased with her chosen being abandoned like this and she granted him safe passage to a land named Albion, to assist her Sister's chosen heroes in their upcoming battle if he chose.
Harry is the (preferably) same age as Sparrow, grows up with Him/Her and is part of the Gypsy caravan after he was found out in the snow the winter before Sparrow was shot.
Harry, or you can give him a different name, still has his magic, but he is not able to use Albion's version, Will, at all. It tends to back fire on him and result in explosions.
Theresa is able to teach harry his version of magic to an extent while he also knows how to use other weapons.
3a. go with a wide variety of weaponry rather than the few classes they have with the Fable franchise if you wish, there is a wide variety that you can choose from.
3b. Don't over power him, try to keep him much like Sparrow is, let him progress through the story, gradually getting stronger with every challenge delivered.
As Fable approves of the different sexual relationships, it can be whatever you choose, he could pair up with Sparrow of either gender for all that it matters.
Harry will go with Sparrow and his/her dog on their journey to avenge Rose and save Albion from Lucien.
At some point I can for see an argument that drives Harry away, preferably before Sparrow's ten year imprisonment in the Tattered Spire, that leaves Harry feeling guilty for letting the other go off by themself with Hammer.
Blaming himself, Harry could possibly throw himself into bounty hunting, or working with the stores around Oakfield, building and expanding even using his own magic to help with the crops growth. Not realizing that Sparrow would return to there in the future.
To continue on from Sparrows return with Garth their quest to Bloodstone, Harry could get separated in the Marsh, and wind up in a portion of Oakvale courtesy of the gate malfunctioning.
While in Oakvale's ruins, (you design them from the tattered view you see of the lost city) Harry could encounter anything, Oakvale could be a crossing point between his home world and Albion, or it could just be a forsaken place.
9a. No matter what, Harry MUST go through some form of character development, whether it be acceptance of something he had done during his ten years: possibly a bounty hunt gone wrong, an accident he wasn't able to help with, or a fight with Hammer.
9b. or finally forgiving himself for abandoning Sparrow before the other vanished.
Let it take however you want for Harry to get out of there, so long as when he does get free, He talks to Sparrow.
Lucien has to use Harry as a hostage, whether it be, him getting captured at the beach, or being taken at the top of the Guild Tomb. Whatever happens after that is your choice, let Sparrow choose their wish, and go on with everything else.
Note: Harry does NOT have to return to his home world unless you want to draw out the story longer. Try to stay following the story line of the Game itself, only add in Harry's magic when needed. Have at and enjoy~