An outline, shared in Real Time by 2 (or more) people over the net, where row-locking allows all people to update simultaneously (though they don't have to, it can be asynch). With the results saved as Structured Text in a Wiki page.
See Wiki And Outlining.
You'd need to be passing around changes to the outline as transactions (via Jabber?), rather than passing the outlines (e.g. OPML) themselves. I think Groove works this way. I think that means you'd need to "extend" OPML to have an ID for each node in an outline.
Analogy: crossing QuickTopic (adhoc private shared space) with UseNet (hierarchical discussion) with Wiki (can repeatedly edit earlier material, scope is a single hopefully-coherent document) (document/thread lives within a larger space)...
InfoDepot, in its beta-that-never-got-released Windows version, supported a feature like this. But over a LAN, not the net. And there was no macro-environment for a document to live within. It treated each node as an objects, locked and broadcast at that level.