With thanks to Ezra for the idea...

Structure:

In the top thread the zeroth author establishes a simple, young world, with a few laws or principles under divine protection, and perhaps some incidental starting conditions. All participating authors (and all readers) can read this. There's also a copy of the rules and a queue to track the order of upcoming authors/characters

The zeroth author starts a thread for the First Age, describing the initial conditions. The first author's character is now god of this world, and they can enact pretty much anything they want, as long as it doesn't contradict the principles. The zeroth author describes the world reacting. The thread is hidden from all authors in the queue.

After some period of "time" the First Age is over and the Second Age begins. The zeroth author steps aside and the first author takes over as host; they start a new thread with the current state of the world, and the second author comes in to write the new god. Much like before, the new god can do anything that doesn't contradict or reverse any previous god's miracles or the world's principles.

And so into the Third Age and beyond, trading off roles. No god can contradict or reverse any previous divine actions, except that if they can show that the old action doesn't matter any more, in which case cleanup is allowed. Authors gain read access to the history when they go from writing the god to hosting the world. After their turn hosting, they can choose to keep reading or to get back in the queue to write the same or a different god (and lose read access until then).

In the event that a miracle or principle becomes obsolete, no longer affecting the world, a god can clean it up; if they remove a Principle in this way then they must replace it with a new one. The top-post of an age should contain most (visible) relevant information about the state of the world and any active miracles; the hosting author will be able to provide clarification as needed during the age, and in the event that a miracle does get forgotten and overridden, the older one is dropped; it must not have been salient enough or forceful enough to stand up to time.
   
Individual plays of the game will have their own player queues, and each can be a section of this continuity. For that section any rules posted there take precedence over these.

The following First Principles apply even before a god has created anything:

What has been destroyed is gone forever. A being's mind is theirs alone. The past is inviolate and unchangable.