Seven-card stud. All hole cards are wild, although they have to keep their natural suits. This is because if a con committee tries to keep anything secret, vast numbers of rumors will surface regarding what they are actually doing. (Only the hole cards are wild, not the like-denomination face-up cards.)
Because anything that happens at two consecutive Worldcons becomes traditional, any two consecutive cards in your hands rank your hand as a straight. (You must keep your cards in the order they were dealt. Of course, since everyone has two hole cards to start the hand, everyone starts with an ace-high straight!)
Any hand that does not include a full house has to match the pot as an attrition clause, because it failed to make its room block. (You can use a different set of five cards to make the full house than the hand you use to try to win the hand; however, your wild cards can only have one value in the end, so you can't use them as one value to fill the full house and another to make a straight flush. If multiple players have to match the pot, they all put in the amount that was in the pot before anyone else had to match it.)
Since somebody has to actually run the convention, players are not allowed to fold. And since no matter how good a job you do, people will moan and complain about it, nobody can win. After the hand is over, redeal and play another round. (You can't win; you can't break even; you can't get out of the game.)
The horrible thing is that, except for the last rule, this game appears to be at least theoretically playable. You'd have to be crazy to try it, which makes it ideal for conrunners.