Suggest that an optional "extensions" element for an
"org.eclipse.core.resources.natures" extension point could
identify the file extensions to which an auto-add-nature could
I think natures are too restraining here. While it's used in most cases, there are other ways an editor can configure a projet or its own enablement (preferences, project settings, a builder, starting a Run configuration, show a wizard...). I believe the diversity of possible actions makes it almost impossible to find a generic solution to this problem that can cover all legacy and future stories.