All, at once.
It's not really a priority, but toying with the directory listings I realise that it would be nice to be able to change the template. I customised the error pages before, but having to have a handful of pages that are essentially the same was sort of inelegant. Plus there are situations when the server generates errors that are not allowed for error handler. 500 comes to mind for one thing. CGIs might generate others which may be up for to hiawatha to handle when using TriggerOnCGIStatus. Which, of course, is helpful when mixing dynamic and static pages.
As is, there's simply no way to consistently change all the error pages. Plus using XSLT is simply elegant and consistent with the other server generated pages (i.e. directory listings).
As I said, it's nothing terribly pressing. But it would be a nice feature to have, so you might want to look into it when you find the time. No reason to make a great webserver even better... except you having reason not to or lack the time, of course.