I think Jon @jonm clearly knows where’re the “Gordian knots” today?
I explained one of the important ones some months ago as:
… the logic behind the IfcDoc tool makes the IFC schema and the IFC schema development more dependent on its data modeling tool which today has issues and buildingSMART International has started to develop it again based on UML, but in reality, UML will not solve the dependency barrier, likewise. -
Source
Personally, have waited to see what would be IFC after redesigning it based on UML
The second vitally important part is related to new IDM called IDM Configurator/Toolkit
I saw some good improvements which come from ISO19650
The third vital part for me is a schema and also .ifc file that is federated/separated
For instance, IfcControl is a separated schema that integrated with other schema parts
It seems that STEP 2017 lets we have a lot of .ifc files and integrate them together