It would be nice to be able to override a single template without keeping a private copy of the entire template directory.
a setup option like
templatedirs => [ "my/dir", "/usr/share/ikiwiki/templates" ]
ought to do the trick.
Now that I look at the source, I see that ikiwiki already falls back to the global dir when it cannot find a template. For me, this is good enough. And it is even documented in the man page. Sigh. I guess this could be considered done.