I don't have the time to read over everything that's been discussed so far, so these points may or may not have been covered already, but I thought I'd pop in and offer a couple of suggestions based on my participation in the previous CWBP.
Give a thought to licensing and how it will affect both the creators' rights to their own work and their ability to use graphic elements. The previous project was licensed under the Creative Commons BY-NC-SA, which had a couple of unintended side effects. For one, the license of the very useful cgtextures.com website specifically forbids the use of their textures in CC-licensed work. Although I believe that is based on a misunderstanding of the CC license (which is erroneously described as "open source" on cgtextures), the fact that it is specifically mentioned there means that those textures are disallowed in maps for the original CWBP. The second problem I perceived was the combination of the Share Alike and Non Commercial clauses, which forbids a cartographer from, for instance, selling prints of their own work because the Share Alike clause causes the Non Commercial clause to apply to all derivative works. By way of full disclosure, I am not a lawyer, and there were some other participants who disputed my analysis of the CC license's language. Nobody presented an argument to induce me to change my mind about it, though.
A modified license based on Creative Commons but tailored for the project might be more appropriate. Also, including language permitting The Cartographers' Guild to use the source material for publicity and fund-raising purposes would also be a good idea. An individual cartographer could choose to include such a term in their own licensing or not, but if it's not present at the top-level (the base map, at the very least), then the option is, technically, closed to all subordinate materials. Several of us put such licenses on our own maps, but since NeonKnight never actually released the base map with such a statement, those statements are of questionable legality.
The other thought I had was to establish and secure a wiki early in the process. In my opinion, what killed the first CWBP was the spam vandalism on the wiki. At the very least, frequent incremental back-ups of the wiki database would have prevented us from losing so much work.