The only way for this to work is by having each collaborator work on one part of the campaign.
There's brainstorming to determine the overall plot, bad guys, mcguffins, etc. Those could be made by polls -- first a thread to ask people for ideas, then after a while, a poll with a dozen of choices from the ideas that seemed the best. That's how the Creatures By Poll worked at the CC board.
Then, once the plot is chosen, brainstorming between collaborator to decide on the episodes. E.g.
►Introduction adventure: level 1-2, town under attack by bandits. Unrelated to the main plot, except the bandit leader's second-in-command works at the behest of the main BBEG and organized this attack as a distraction.
►Reprisal raid: level 2-4, PCs must find back something valuable that disappeared during the attack. First hints that there may have been something fishy behind the bandits. PCs enquire and eventually find out mcguffin was taken on a ship.
►Nautical adventure: level 3-5, PCs try to catch the mcguffin's ship and get annoyed by pirates and other sea hazards.
And so on.
Then once the layout of the campaign is made, episodes are given to individual members, who start developping them. To avoid railroading too much, several plot hooks must be used, each allowing to move on to the next adventure.
Every time someone is finished with his adventure scenario, then everybody reads it, comment on it, make suggestions, etc. A few continuity guys look for plot holes with other adventures. More plot hooks can be added for easier transition between modules. Once the scenario is agreed upon, stat-block guys prepare/proofread the NPCs. If some have volunteered, art guys can start making illos of NPCs and maps of locations.
Once everything's finished, layout guys turn it into a full-blown PDF.
People will be doing that in their free time, so you can't expect them to do as much work on their own as a professional writer. You need to cut it into tiny chunks so that everybody can do that easily in his spare time. Since it's collaborative effort, it's not a problem -- instead of one writer, one editor, one artist and one mapmaker, you'll have dozens of each.