Funding would be the biggest hurdle. Without the funding, problems with dedication and discipline arise. And you're right that agreeing on a story would be a major issue. Now if someone had a story that was well-liked and had a lot of money, that person would be the de facto director and would need to make it happen.

I don't think a purely collaborative effort would work because everyone would expect control roughly in proportion to those resources given.

There is a lot of talent to be tapped in terms of editing and effects on this site and others. There are impressive model makers on other sites. Any number of us know people who are actors or involved in movie making of some sort. But quality control would be a stumbling block even if we had the money. Even the best of fan films suffer from some poor acting and a lack of polish. Despite the level of talent we can probably find, it would take a lot to make sure the film satisfies our expectations for its quality. Maybe I underestimate the talent that could be found - I hope that is the case!

The elephant in the room is lack of authorization by Lucasfilm. GL has been great about letting people make fan films and edits as long as no money is made. One would hope the same standard would apply if fans wanted to create an alternative vision of the prequels.