Baron Munchausen is a storytelling game, the rulebook for which I have uploaded in PDF at the link. I think it would be fairly readily adaptable for forum play if we can get three or more players to cooperate. In brief, we take on the personae of eighteenth century nobles and take turns telling tall tales of a certain characteristic aesthetic, which you will get the best picture of if you read the little book. (It's 26 pages long, and that's including both covers, an appendix, and heavy illustration.) There is a system to make this more interactive by allowing challenging interruptions, which we will have to allow for by submitting perhaps a paragraph at a time and waiting for someone to either issue such a challenge or post something along the lines of "do go on, my dear Baron". These challenges are limited in the rules by issuing everyone a number of tokens, which number I propose we double.
Optionally, to make it more thematically appropriate to the fledgeling forum, we could adopt personae other than those of 18th century nobles; for my part it would be fun to play in character as Princess Kiawen or Kira Soiah-lei or the Archduchess of the Peninsular Coast (but we should be all in more or less the same setting, although our characters congregating in Milliways is not out of the question).
ETA: Got an informal C&D about hosting PDF.