General developer forum

Some plugins support subplugins (e.g. assign, book).

Most plugins don't have in place support for subplugins.

Local plugins are a bit of a dumping ground for putting plugins that don't really fit anywhere else.

If you're trying to alter the behaviour of the wiki module, you may find it hard to do without making changes to the code of the wiki module. There are a few things you can do without changing the code, but they are mostly limited to adding items to the settings menu, injecting javascript to alter things on the page inside the browser or writing new PHP scripts that retrieve/alter wiki data directly in the database.

See https://moodle.org/plugins/local_wikiexport for an example of what you can do to extend the wiki module (via a local plugin).