It's an interesting idea, but I'm unconvinced that a community effort can produce results (in this case, films) superior to that of a highly skilled artist. Has such a system worked in any other medium? We have the technology available now to collaborate on books on a massive scale, but are there examples of this happening and succeeding with results that rival Kurt Vonnegut or F. Scott Fitzgerald? Similarly, a GitHub for music is certainly feasible with today's tech, but I don't think this recipe guarantees results better than a small dedicated group of artists like Radiohead or Nine Inch Nails or <insert favourite band here>.

Don't get me wrong, I think the system you're proposing would be fascinating and could produce interesting material, but sometimes great artists produce their best work when every aspect is under their control.