You may or may not be aware of a genre of puzzle game on the internet where you reach the next level by using clues displayed in the URL, images on the page, names of those images, occasionally editing those images, and a few other techniques to correctly guess the URL of the next level. They're hard, and they're fun to solve. I think having a group of people to collaborate on solving the riddles and giving help to people who need to catch up would be pretty fantastic.
There are two of these games I'm most familiar with.
EBONY is one of them. It features forking paths, a storyline that may or may not make sense, and it's the easier of the two. I haven't beaten it, but as a group we can probably finish it by the end of the year.
Then there's notpron. It is extremely hard. I have not made it very far at all. An entire 34 people have been confirmed to beat the game. We can probably do it, but it's going to be something that takes years to do.