I could not find this wish in the forum in spite of many searches using different criteria. But I know that the ability to save Revit to a previous version has been on many peoples' wish lists since the beginning. And I understand the reasons it would be difficult to implement this in the way that, say, AutoCAD is able to do it.
But suppose we had the ability to copy & paste content between different versions of Revit running on the same platform. This should be an adequate workaround for most situations, except where some features or geometries are incompatible from one version to an older version. So that is my wish: to be able to copy & paste between different instances of Revit.