@Emerald: I think it would start with a vague sketch of how different segments of the map are connected, randomly assigning segments with coordinate bounds to different mappers to elaborate and block out one by one, then testing and refining the overall structure/gameplay, passing on to an environment artist to realise an agreed theme, someone to work on optimizing it, someone to polish it off and someone to build the final package. Each step within specific timeframes. There would be room for texture and model artists to develop something early on, too.
I suppose it would be a bit like a simplified version of the production process professional teams would follow. With enough volunteers we could end up with the biggest map ever made for Q3!
@raspatan: I'd settle for a smaller collaborative effort if there's not much of a response.
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