Ok well as we're getting a themed week it'd seem a waste to not take advantage of it so I had an idea which kind of spawned from bonopis idea. So here it is...
Me and a few organisers find a creepy location with lots of rooms and possibly floors and we invite anyone who wants to come and they enter the building guarded closely by level 50s at the doors. Then once we start, all the people gather into an area where "the host" will give everyone a little opening speech and announce he's got a little game for everyone.
He'll announce that now everyone has entered they may leave when they want but there will be no re entry and the last member standing in the house will gain a prize (though we'll have to rely on matureness that everyone doesn't kill eachother). But to make things fun and twisted there will be tasks given by the host which involve puzzle solving but also death. Basically the point is to put lots of people in a house and the host will mess with their minds, turn people against eachother and pretty much spook them. If anyones seen "saw" it'll be like that.
It's just an idea currently but if there's any interest then please reply, it's just something differant and we could really mess with people.