I can't remember a whole lot about Fable 2 and 3, but I do remember a lot of third party stuff. A modified KoreVM and LUA+ and Havok, less in-house studio and more studio-for-rent stuff. I suppose it's too much to wish for, a game that allows unlimited access and editing of scripts and assets in entirety as opposed to limited and difficult access to one or the other.
The magic formula for butt-tons of mods is a game that offers easier solutions to modding, that allows enough of the game to be altered in a manner that doesn't require Neo's Matrix vision, opening modding to a large enough community that you get enough activity and interest to keep the momentum moving for a decade. Sad to say that Fable, great game that it is, and especially its successors, of dubious quality and flavor, just don't satisfy that magical formula.