For one type of plugin, see [[todo/PluggableRenderers]]. 

A plugin system should ideally support things like:

* [[todo/lists]] of pages, of mising pages / broken links, of registered users, etc
* a [[todo/link_map]]
* [[todo/sigs]]
* [[pageindexes]]
* Wiki stats, such as the total number of pages, total number of links, most linked to pages, etc, etc.
* etc

Considering ikiwiki plugins, one idea I have is to make the [[PreProcessorDirective]]s be a plugin. A setting in the config file would enable various plusins, which are perl modules, that each provide one or more preprocessor directives. 

Since preprocessing happens before htmlization but after a page is loaded and linkified, it should be possible to use it to create something like a link map or lists, or a page index. Page inlining and rss generation is already done via preprocessor directives and seems a natureal as a plugin too. 

Note that things like a link map or a broken link list page would need to be updated whenever a set (or all) pages change; the %inlinepages hash already allows for pages to register this, although it might need to be renamed.