How about creating some kind of short story? It could start off with someone posting the opening line, and then posters could contribute to it a line at a time...until the last line that would be in post 800. Each poster could keep contributing lines as long as they didn't put up two posts in a row.
Maybe, before starting, we could decide on several lines, objects, etc, that would have to be included as part of the story or something.
Anyway, just an idea, and not sure on what the general theme might be for it. And wouldn't necessarily have to be a story. Could just be something that people could keep adding...and adding...to...or something...I'll have to give this a bit of thought!
EDIT: How about a kind of improv play? We could work out between us characters for the 'regulars' to be. And the 'lurkers' could just make up their own characters and place themselves into/out of any given moment of the play. Not sure how all this would really work, though! And suppose the problem with doing a story is that you might not know what on earth was going on unless you'd been following it from the start! Um, think I might just have thought myself out of this whole idea!