Adding or theming argument defaults

Is there a way to add a new type of default view for an argument, beyond the built-in 'Return Page Not Found', 'Display All Values', etc?
Or failing that, can it be drastically themed to return something different, from the definition of a new argument type in a module?

I'm trying to replicate the galleries made by image_gallery.module as a view, and I need to show a list of top-level galleries for the URL with no arguments.