Dumb idea perhaps but here goes:
The week before you want to in fact run this game, call off your usual session and have everyone go to the pub instead - tell them the pub night itself is the 25th-anniversary celebration.
Take pens and paper with you. Once everyone's there, start talking at some point about what would be the greatest dungeon ever, and get people to throw in ideas. (at this stage, the drunker the people are, the better!)
At the end of the evening, scoop up the papers and take them home. During the week, try to make some sense of it all.
Then for the actual anniversary session the following week, run whatever they designed.