The problem with this is simple. In order to do it properly, people need to all be willing to put in a vast amount of time- if everyone doesn't make that commitment, then you'll end up with something that is a bit rubbish. It might be interesting, cos it was a community thing, but it'll be a bit rubbish and will have wasted everyone's time. This is only worth doing if the end product is going to be at the same standard to what we can create on our own and, frankly, I'm not sure if that is possible.
I really suggest that, before you take this any further, you thoroughly read through those threads I've linked to above. The Organ Lottery project was potentially fantastic and we came very close to realising it (if you search 'organ lottery' you'll be able to read several excellent scripts that were written for the project). Personally, having led that failed attempt, I don't think it is going to work. To produce a short film requires time and money (unless you're willing for it to not be of particularly high quality) and getting many different people to do input that for a community project is a very, very tough ask.
EDIT: I just re-read some of those threads and we actually came up with some great ideas for how to do this. If you are going to try again, Absolution, then I recommend you look at these ideas which we came up with and then voted over in a poll, as they could all work for this sort of thing:
The Organ Lottery 'In a world that is starved of usable organs, [X Number] people across the world wait with baited breath to see whether they have been selected as one of the lucky few to receive a transplant. The build up and fall out of this momentous wait will change all their lives, for better or worse.'
The Cure 'The world has fallen victim to a terrible plague and, with time running out, only 1000 people worldwide can be vaccinated in time. Across the world [X Number] people wait to find out whether luck is on their side, or whether they must accept their fate.'
The Uterine Plague 'A uterine plague has swept the globe rending all women unable to conceive. Twenty years on and civilisation is on the point of collapse with the population plummeting and disease and disorder rife across the globe. Through this setting tales of desperation and hope emerge.'
Survivors 'Finding unity in the voice of a far off radio broadcast small bands of survivors struggle against a zombie plague and its victims to reach a rumored refuge.'
The Geriatrics 'Abandoned by the country they love, and without family to help them, five elderly strangers seek the support they need and the revenge they crave.'
Going Viral 'A debilitating flesh eating infection threatens to destroy the last remnants of the human race as people all over the globe struggle to survive and regain control of the planet.'