There are many occasions where the usual <noinclude>, <includeonly>, <onlyinclude> tags are not enough. It is often the case that something needs to be transcluded or substituted once, but no more. This is the case with templates that are transcluded in other templates, but it is not wanted to be transcluded again in articles and such.
I propose tags <includeonce></includeonce> that will wrap sections that must only transclude/subst once. Perhaps, they can be converted to <noinclude>, after the get transcluded/substituted.