As for a heirarchy - I don't know if it'd be wise to have a rigid structure like that. That is, if somebody is seen as 'in charge', and that person is deemed to have too much control, or alternatively is having too much say over where the project is headed, people won't like it and will leave.
I think it'd be better to have a looser structure, where votes are taken reguarly. As such, nobody has 'more power', but rather more responsibility. That's how these things tend to work in the long-run - if it's based on a 'power structure', projects crash and burn - a la millions of other similar OpenSource projects. Power structure's only ever work if the people below are getting paid a wage
In my opinion, it'd be an idea to have people put in charge of relevant sections or segments of code (or admin) - and everybody should have equal weight when it comes time to vote on a project change etc.
Basically, nobody will tolerate somebody bossing them about outside of work. Tis the interweb afterall
For example, has a vote been taken to say that a game is what the contributing dev's are willing/happy to develop? Is it too early to take a vote on such a big decision?
Edit: It might help to point out at this point, that I'm all for the game idea - I just think it's important to make sure everyone else is behind it! Would especially love a Mafia game
What about modding an already existing game? Or writing for the UT engine or similar?