In reply to In that case, I’d …

In reply to

In that case, I’d probably opt for a custom taxonomy. Anyhow, I had another super-brief look and it _does_ contain that line of code (and a similar one for tags), plus code to register two custom taxonomies aptly named “categories” and “tags.” (I believe WordPress’s built-in taxonomies are “category” [singular] and “post_tag.”) Plus a whole lot of code I think might not be needed, strictly speaking—think admin pages, or widgets.

Using your site-specific plugin, you could fairly easily reregister these taxonomies, display terms underneath posts, and things like that. And everything would still be there, even if it looked a bit different. (With the help of a child theme, you could even add your own archive templates, and so on.)