"Sign up should be always on a first come-first-serve basis. Simply lay out performance areas with a sign up sheet with a breakdown by hour or half hour performance. If an artist arrives out of the blue they should find a stage at a moment's notice."

Been there, done that, Woodford Festival - all slots filled within 5 mins of the blackboards being open for each session.

I like the minimal amplification idea.

Some good ideas, but otherwise, sadly mostly not practical. Any hint of rain, and my expensive instruments will not be there, might take a tin whistle...

You certainly will have a lack of vendors, goods and food.

Others have already said most of the rest.