Shared containers - categories and containers that exist in multiple hierarchies

The basic idea behind this proposal is that a set of categories in a containeri often need to be 'shared' between multiple sections of the site. The easiest way to explain this is with an example. Say you have a container called topics, that has a whole bunch of categories that are used to categorise posts by topic - e.g:

And say that you post content to two separate sections of your site - the news section, and the blog section. However, you'd like to use the same topics to categorise your news posts and your blog posts. This is your only option for achieving what you want at present:

But you don't want it to be like this. With this structure, the topics container only lets you browse by categoryi with the news and blog posts all mixed together. What you really would like is to be able to browse only the news posts by category, and only the blog posts by category. You'd like your unified tree of categories to be shown in two places, and to only show posts of one type in each place. Example:

At present, I only have a hazy idea of how this would work. It would be quite complicated to implement - it would probably require a separate contrib module, perhaps called category_share. Shared categories and containers would initially be created just like any other categories and containers - that is, as node types, with a single page from which they're accessed. But then category_share would provide a special callback path for accessing a category within the context of another category - in effect, filtering one category by another. For example:

This path would simulate the node page for movies, but would filter its node listing by the news category. The page would look very similar to the 'real' movies page (which would be at node/6), except that there would be no 'edit' ability, no 'comment' ability (and no comments displayed), and no 'add child category / container' links. It would just be the body of the node, and a specially filtered category listing (this can easily be achieved, with a custom call to node_view()). Adding new categories to this structure would be managed centrally through the 'real' page, or through the admin interface.

The tricky bit, of course, will be integrating such a system with category_menu and with category_display. This will be difficult, but it will be possible. If it's done properly, it will be able to give the impression (to end users) that shared category pages are no different than 'real' category pages - especially if they're given URL aliases. Breadcrumbs, table of contents links, and navigation links will all work together to give this impression.

This is something that a lot of users have asked for. And when it's available, it will seriously open up some new doors for the category module. But developing it won't be easy, and I for one am not planning to do it just yet.