Billy Corben - the documentarian who made "Raw Deal," "Dawg Fight," and "Cocaine Cowboys" - discovered a formula to help predict if he should move forward on a new project, and its probable success in the marketplace.
He calls it R.A.S.
It stands for:
Relevance means: Who gives a shit?
Relevance shouldn't be confused with being topical.
A topical subject is the like ripping from the headlines: the consumptive story of the day, which has little lasting power.
Relevance means WHO would want to see it, and why.
(Hint: the niche makes you rich.)
Access means: Can you actually tell this story?
Do you have the access to the subjects, the locations, the archive material, or anything else you need in order to tell the most compelling story possible?
You might have a great idea for a documentary, but if you don't have access to the subject, then you won't be able to tell the story.
Sometimes, this makes for great entertainment on its own ala Nick Broomfield (where his movies are about the inability to gain access to his subjects).
To quote Corben:
"Your job is to find a good story... and not fuck it up."
Style means: Will you be able to tell this story in a new & compelling manner?
Relevance is important, access is important - but if you can tell that story in a style which attracts as many eyeballs as possible, in a novel form that keeps people talking after the movie is over - then, buddy, watcha waitin' for?