I've come late to the thread (still waiting for Satin Snow, grrr ...) but I'd like to make a couple of suggestions.
Magnachrom is quite right, IMHO, to point up the value of synergy. It can lead to considerable growth, but also, it's really the only way to avoid the main pitfalls of vanity publishing - the cost of quality reproductions, and low public diffusion. Photography requires good quality repro, but LF (correct me if I'm wrong) demands exceptional quality just to avoid disappointment.
However, I'd suggest that before marketing issues can be decided, one factor needs to be considered ... which should have prominence in the theme of the project, the subject matter, or the medium?
The reason I stress this is that Flickr does an excellent job of attracting mass viewing where the subject matter has public appeal ... and if you're not keeping an eye on public appeal, the viewership may turn out to be (unsurprisingly) low. Web-viewing has its limitations ... most evidently in the case of LF, but again, subject matter or medium?
The idea of four seasons is a beauty (there's an admirable example here:
http://nelsonfoto.com/v/showthread.php?t=7811) ... but it takes a long time !! And in Britain, a one-day shoot is bound to be beset by weather issues in one quarter or another, if not all together. I like the idea of working the coast, but wonder if the distribution of LFers here is up to it. I also wonder if the geographic distribution of photographers here could suggest a theme ... setting up a Flickr group map (it can be private) would be a quick and easy way to test out commonly occuring interests across the UK, with images appended to the locations where they were shot. In other words, find a theme by analysing what we already do wherever we are - and then push it further with a deadline.
Another theme that crosses my mind is "Working Britain" - I think there was once a photo book by this title. Much as I enjoy doing landscapes and graveyards, I one day hope to get into serious environmental portraiture - can't say I know any novellists or poets, but I do know a few mechanics and farmers. Quite a challenge, but a group project could push me to it. Otherwise, there are all the common genres from flowers to architecture, from food to fashion, etc ...
At the end of the day, what each of us does well is probably what each of us would do best ... the diversity among large format photographers in the UK would be a theme all its own.