The ability to "Load As Group" via the Revit API. This is currently available in the UI.
It is said in this forum post that it is logged as an internal wish item CF-987, but there was no "Revit Idea" for it. So please up-vote.
https://forums.autodesk.com/t5/revit-api-forum/load-as-group-revit-api/m-p/6408663#M16968