I was thinking, and we can allow multiple tags per topic by virtue of forum trees.

E.g. a topic in A > B > C, would have A, B, C as the tags.

We could set Composr to automatically self-organise the tree based on what tags people are using. The optimal smallest tree for the particular tags people have set.

The available tags would be all forum names added in a particular group, but the actual structure would grow out automatically.

For this we should change the admin-UI, as it would be totally confusing. We should have a simple add/edit/delete tag system, rather than full forum structure management.

I am really excited about this concept as it solves some long-running problems. People want rich categorisation of forums, but it so often leads to these messy situations where topics are really spread out (especially for quiet forums). Also managing structure is a real hassle. So we can have everything together by default, while still providing full classification, full filtering capability, and maintaining a rigorous underlying forum structure that can be toggled back on later should the forum become very busy.

New topics would take on active tag filter as default tags.

I have implemented some of this for a client. It's more of just removing forum structure and replacing with a single list of topics and tags, so there's lots more that can be done. It's a great start though. Hopefully I can merge in a patch before too long.
EDIT: Patch may now be outdated.