Are you just thinking of making a forum, or a full blown CMS? What's your role in this, are you a great coder, or a great co-ordinator, a marketing man?
I'm not trying to put my foot through this thing, but you've got to wow people if you want them dedicating hundreds of hours of work for free. You might end up drawing a load of newbie learners into it and you all learn a bit on the way, which is nice, but if you're looking for a product that's going to challenge the stuff out there that's already pretty damn good and has masses of 3rd party support you'll have to draw in people who already know what they're doing. These other people are already fixing their bugs and moving onto complete rewrites already (Joomla 1.5 on horizon), you'll have to write loads just to get to where they were 2 years ago.
Can you put something down on paper that'll make people want to invent a recipe for cake rather than eating the one already in the oven? Are you happy with the other option of a team of learn-as-you-goers, which can be great, but can also be hell.
How many people do you think you'll need on this team, and how will tasks be designated? Have you got any designers lined up. The people of the internet are fickle and even if you've got a body to die for they won't give a damn if your face looks like a smashed crab.
Wow me.
