I'd like the ability to block external links from anonymous users, or from
untrusted users. This could work by generating the HTML for the new page and
comparing it to the HTML for the old page, looking for any new
<a> tags with
href values that didn't exist in the old page and don't start with the URL of
the wiki. Comparing the HTML, rather than the input, allows usage with
various types of input formats, and ensures that a template, shortcut, or some
new plugin will not bypass the filter.
This would probably benefit from a whitelist of acceptable external URLs.
This may actually form a subset of the general concept of content policies, described at fileupload.