The way I imagined it would work is using subcategories. These can’t be created by users (at least by default) [as opposed to tags, which can], however perhaps that is a good thing as I imagine we don’t want the sub-categories (or tags) cluttered with projects that receive less than a page or two of replies. It should be easy enough to get hold of a moderator and ask for a sub-category to be created as and when needed.
This is what discourse has to say about tags:
Tags are a useful alternative to categories for some forums. For example if you were running a music forum, being able to tag a topic as “Hip Hop” and “Electronica” would be useful. If you were using categories you’d have to choose one or the other.
Which also makes me lean towards using sub-categories, as I can’t imagine a topic being posted to more than one project.
Here’s some screenshots of how the two mechanisms work:
Tags can be selected from a drop-down scrollable menu on both the Categories and Latest page:
You can also select a tag from any list of topics:
This is what the page looks like once you’ve selected a tag; note that you can simultaneously narrow a tag selection by sub-/category and that you can also see which category each post comes from in this list of topics:
Similar to tags, sub-categories can be selected from a scrollable list. However, you must first select the category from the Latest page:
Then select the sub-category:
This list of sub-categories cannot be accessed from the Categories page. Instead, the sub-categories show up beneath their respective category:
15 sub-categories are displayed above, which is also the total number of subcategories the Local Teams category has. I’m assuming discourse takes a sane approach to this (as opposed to displaying an infinite number of sub-categories on the Categories page), but I’m not sure. Perhaps we should check.
You can also select a sub-category from any list of topics:
Each sub-category must have a different color to its parent category. Discourse displays both colors when looking at a list of topics (as above) so that you know the category is actually a sub-category.
This is what the page looks like once you’ve selected a sub-category; as it doesn’t also have to display the category of each post, the appearance is slightly less cluttered:
A final point to consider is that it is not possible to create sub-sub-categories in Discourse (as far as I’m aware).
For reference, the tag screenshots were taken from Twitter’s discourse and the sub-category screenshots from Ubuntu’s.