It would be useful to have an option to merge Ref attachments as Cells. Sort of equivalent to ACAD's Bind XRef. Similar use cases.
Graphic Cell - the Cell would be placed in the active model. Nested Refs would be converted to nested Cells. Option to save to external library or active dgn as new model with Ref attachments*
Shared Cell - the Shared Cell would be saved to SC Hidden* definition model as a Cell**, and replaces the Ref attachment in the active model.
*Ref attachments would be re-pathed as new models in the active dgn. Default.
**not accessible for Open for Editing in the Cell Library dialog.
***Presumably, any nested Cells (which are Ref attachments in the external Cell library) are either converted into nested Graphic Cells in the SC definition model. Or converted into separate hidden definition models that are attached on placement?