As anyone who's been around here for any length of time knows, substantial topic drift is almost a certainly. About all we can do is to try to stop radical departures from the theme of the moment and to otherwise try to slow the drift a much as possible. But it will happen.
Here's the experiment I'm proposing:
We choose a topic theme and someone to post the opening message. Then the next person responds to the theme of that message but also throws in a little variation on the theme, maybe a subtle tangent. As the messages accumulate, the message theme will slowly part company with the original message 1 theme. In a sense, a "ring species topic", where the neighboring messages are closely related, but the beginning and ending messages are far from being related.
The choice of individual member posting the next message will need to be strictly controlled (no multiple replies to a message). This could be by having a pre-selected list of who posts in what order, or by having a chosen moderator (probably not a forum admin) pick next poster. Anyone posting out of place will have his/her message content deleted.
Message length will need to be limited to fairly short. Connect to the theme of the moment, do your little variation/tangent, and end the message. No tangents off of tangents off of tangents.
We can have a "peanut gallery" topic, where judgments can be made that a poster has made too large of a jump - One must not abandon theme within a message (do an illegal tangent so to speak).
I would encourage the participants to put care into their messages - It is not a race to see how fast you can respond.
Another variation might be to select two far apart themes and see how many messages it takes to connect them. But I think that should be a second experiment.
I may be interesting, it may turn out to be totally stupid. Whatcha think about trying it out?