Well, the only maps I can think of that were made by the community and turned out really good are the various popular CS maps. And, to my understanding, the only reason they turned out so well was the fact that they were in fact tweaked for years by the community as a whole, which served to function as alpha testing. No matter how dedicated the modder is to making things perfect, it takes countless hours to tweak games like this to perfection.
Only way i could see it happening is if a situation like CS happens where the whole community works together to alter the maps after hundreds or thousands of hours playing. And if this were to happen, I wouldn't really care about the theme so much as having a really epic map.
Indeed CS was modded but it wasn't the only game that was modded. For example take a look at Enemy Territory, with tens thousands of followers and after 5 years and still going. Custom maps were created, various mods were also created , ie Pro, Jyamod, Pub, NQ, even when Punk Buster was not in a mood or for what ever reasons they chose not to expand on various exploits within the game, the community itself moved into overdrive to achieve a cleaner level of gaming. At the moment, slac is the promising side to et, as its anti cheat system is in the wood works and being applied through various game serves and client side. But as you pointed out, ultimately its upto the community itself to pull together. I am confident this could very well follow down that very same path. What has also captured the heart of the gaming community is that server files are being released, thus allowing maximum online game interaction. This in my opinion is what makes and breaks a game. Don't confuse gamers as if they are non thinking entities, remember, there are thousands and thousands of clans out there, others are small and others are very big. So it really does not take that much to swing a vote of confidence from one side to the other. Obviously the devs of Brink realized this and is and was applauded throughout many gaming communities.