While I'm pretty sure nobody here has been asking for Fable fanfic, given that at least some of us have played the games and the rarity of anyone writing such a thing, it has more potential to be accepted than someone writing fanfic of some anime.
As for why there aren't many fanfic games, well some would say there's actually TOO many fanfic games on the site already. This is probably one of the rare writing sites where fanfic tends to not be very well embraced for various reasons. The main one because most people who are writing fanfic don't write very well in general.
Writing something shitty yet still original (Or at least trying to be) is one thing, but writing something shitty that's already based on an existing franchise is failing pretty hard. You're either ruining an already very well established setting or you're not really adding anything better to a setting nobody liked in the first place.
Plus most of the time the fanfic is of a franchise that's cringy anyway (Like cloppers making MyLittlePony romance or some such degeneracy.)
This isn't even touching upon the dreaded "Warrior Cat" invasion this place suffered back in the early 2010s. There were BAD Warrior Cat stories EVERYWHERE. It was terrible. Even today we're still finding hidden WC stories we haven't completely purged yet. It's literally the only form of fanfic that's been officially BANNED from the site.
However, despite all that, it's long been accepted that for good or ill, fanfic is something a lot of people like to write so there's a place for it and depending on the writer, some of it can still be good.
Which is basically what this all boils down to. How well you're going to write the story. As I said, Fable fanfic isn't something that generally comes to mind when someone says fanfic so it's at least a lot different in that regard. Could be pretty interesting. I don't even remember all the minor lore to it.
Also you probably can't mess up Fable more than Peter Molyneux did. (That series really could have been so much more)
Anyway, I'm sure Mizal or someone else can answer some of your other questions.
EDIT: Oh looks like I was too slow and they already did.