I'm a naysayer only because I had a major hand in the first Snarkpit collaborative map that took two years and died at the very last step of development. Everything was done except for weapon placement, but to do that at an acceptable level was, to me, nearly impossible.

The most workable layout to me is a nine square grid, with the middle section done last. Each mapper gets a square or two (depending on participation) and it is passed around in predetermined order where each mapper builds upon the work of the previous. This is done so the map feels connected, even with the different construction styles. The middle section is last so that all the loose unconnected hallways can be wrangled together. Also it helps connectivity, making the map better. I would suggest an experienced mapper for that last piece, cause it is difficult work and not too fun .

If you can't make a playable map out of it I don't see why you'd even bother making it. Gameplay can be preserved by skilled mappers and solid layouts. I'd be willing to help but you guys gotta understand the huge project you're undertaking

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I think examples would help, so here's the first collaborative map. It never had an agreed-upon name but it was the 'snarkpitcrew map' or 'spit' for short (thanks Orpheus... ).

The order of mapping went something like Grimlock, Yak_Fighter, Orpheus, Alien_Sniper, Dietz, Reno, Alien_Sniper, Yak_Fighter. The construction was organic so there's massive positional differences between many of the sections, with Grimlock's like 3+ stories tall, Alien_Sniper's section at Grimlock's first story, and mine at his third story.

Alien_Sniper wanted a chance to redo his section because by the time everyone finished it had been a year or so and he had improved greatly in skill. After him I wanted a chance to redo mine since there was horrible connectivity between all the sections and Grimlock's and my sections had too many unused connections. So I went through after much delay and like tripled the size of my section to span three stories and connect every other section. (T means teleporter)

upper area:

lower area:

Upper and lower sections connect obviously, but I can't really draw that. Connectivity is the most important part so that's gotta be a major focus point of all the sections. Hopefully if you guys do attempt this you won't end up having somebody want to hang themselves trying to finish it.