For the avoidance of doubt, the fact that we have five official settings to play with does not mean that GCP only has five settings.
For instance, Aaron Smalley (nod to Aaron) is working on No Quarter under the Crown, which is based in his world of Aernth, for which we have already published City of Archendurn and Dun Cru. Subject to his real-world commitments, Aaron will probably send further manuscripts GCP's way in the future. What we won't be doing is opening up a wish list for Aernth modules. If someone wants to collaborate with Aaron on writing an Aernth module, talk to Aaron. Likewise I'm not going to chain Aaron to a wordprocessor and make him crank out X Aernth modules per year.
Whereas Terry and I expect to have a wish list for Shadow World modules to be written by people other than Terry. (Because if we don't, I will end up chaining Terry to a wordprocessor). Likewise for the other official settings.
For other settings/source material, like Aernth, which we might call "unofficial" settings/campaigns/modules, I might try to assimilate them into a main setting. An example here is Chris Seal's current work in progress, Caer Glais (nod to Chris). With a bit of joint thinking, Chris and I were able to see ways in which Caer Glais could be adjusted to fit into the Shatterings of Cyradon and have links to the rest of the Cyradon/Mithra mythos. So opening up a different part of the world, a different style of adventure, and allow Chris to press on with his good ideas.
An alternative option, for when integration (like Caer Glais) or occasional publication (like we are doing with Aernth) does not make sense, is the all-in-one setting book or the limited set of books. So if someone comes to us with a proposal for a steampunk setting or a dark faery setting or whatever, and if it is good enough, we might well put them under contract to write it, but such things are not on GCP's mandatory projects list or project wish list, so we're not actively seeking out people to write these sort of products. Yet.
Best wishes,
Nicholas