Just thought I'd revive this idea if it hasn't taken off.
I have an idea of one way this could work. I was thinking that people could make threads in the forum with "template" maps they've created. Basically what these template maps would be is just the terrain/rivers/mountains, starting points, roads, and other map defining objects (like if there was a church in the middle of the map, or important bridges). They would create a thread with screenshots of their template and people could ask to be the one who gets to work on it. The uploader would then have the choice to agree or disagree to send that person the map files (depending maybe if they like the maps that person has already made). Also the original uploader could state certain conditions.
For example I might start a thread advertising this as a template with the condition that you can't extend the beaches further into the water and that you can't remove he church.
As you can see it's pretty barren. Beaches are empty and the midland is empty, but the textures are there and it's still fairly detailed. The person who downloads it can now decide where to place strategic points, roads, where to place buildings and other objects. If they want to build a town in the middle of the map they can. If they don't like the trees on the side of the map they can remove them. They can flatten cliffs if they need beach access at certain points, and do pretty much anything they want with it so long as it stays within the same "theme" of the map (rocky shore) and they don't break the original uploaders conditions (no extending beaches). Once they felt they were finished adding their touches to it or if they don't want to work on it anymore they would share their progress with screenshots in the original thread and see if anyone wants to continue working on it or work together with whoever has already participated to make the final changes so that the map is balanced, not too cluttered, and that textures flow seamlessly.
To make this work though we would need to agree on certain rules before the original uploader passes the map off, such as no changing population cap, no changing starting resources, using default number of points (relic default of 10 territory, 2 fuel, 2 muni). The final thing to agree on would be who gets to upload it. I was thinking we could create a shared Steam account where we could upload maps that were created via collaboration that meet quality standards (balance and detail, that could potentially be considered by Relic to be included as maps in game). Whoever worked on the map would have their names featured in the workshop download page with links to their private workshop maps and have their usernames in the loadingscreens.
Thoughts?