I don't know.
Maybe have a section for video games, perhaps. Politics, maybe?
I think that something that inhibits participation is that threads aren't bumped so that you're not seeing threads that get the most participation. Maybe have an option to see what's been replied to most recently and another option being what is occurring now with threads ordered with when they're created.
You know, instead of being an imageboard, maybe being a 'sound' board. You can post sound clips but not images. When the mouse goes over a little section, it automatically plays the sound. That probably won't happen but I just thought of it.