Would you be able to make these rules in the Scenario Design Forum so people could start generating those maps? Or maybe announce a competition in the near future as a forum-wide Announcement so all these active players might know about it?
If/When you do make such an announcement, you ought to point to those great map-making tutorials you have in that forum, plainly lay out the qualifications for particular maps (size limits, region terrain restrictions, etc), and say what little rewards would be available for those whose map(s) are selected to be incorporated. Then give players a few weeks to generate content and see just how much there will be made!
Also, it was an aside before, but I feel it is extremely important too, that you should have a specific map-generation competition for City terrain. It may be more complicated in some regards, but variation there would be delightful to players in the long run (more customization = happier players I believe).