ETAR is successful because it's a fun, and very relaxing shoot. No score cards to intimidate anyone and slow down shooting. They started with 200 attendees in 1989 so 500 isn't out of the question. As I recall there were just twelve vendors at the first one. Attendance now at ETAR is in the multi-thousands, with about 3000+ registered shooters, last count I had.
The have a kids area...aerial shooting..balloons.
They also have 8 different ranges; of course they didn't start with that many, but grew into it.
It needs to be a place to visit, share campfires and stories; a seminar or two don't hurt but aren't all that important in the grand scheme of things. Make it fun and make it accessable, and of course, affordable.