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.
+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 %depends hash
+already allows for pages to register this, although there could be some
+strange behavior if mixing multiple directives some of which exclude pages
+that others might want to include.
I need to look at the full range of things that other wikis use their plugin systems for, but preprocessor directives as plugins certianly seems useful, even if it's not a complete solution.