This is very interesting!
Although there are all these existing features (various xrefs, worksessions), collaborative work is not as easy at it could and needed to be. In fact, it’s quite a roadblock.
When you re-evaluate these features, please consider doing a system that allows multiple users to work on one and the same file! This is possible in e.g. Revit and ArchiCAD.
As an inspiration: in ArchiCAD, when multiple users share a file, every user can ‘reserve’ a scene object by the click of a button in the teamwork palette, and ‘release’ it when finished (There’s even the option to reserve an object just by clicking on it).
Worksessions are ok, but there are numerous shortcomings. Layouts don’t work as expected (if an attached file contains layouts, I’d expect them to shop up in the active scene), and plugins like VisualArq still struggle to support them.
Actually the main purpose is splitting up a huge scene into parts, less to allow for collaboration. Yes, you can see what another user is doing after refreshing his attached file, but it’s difficult to ‘swap’ an object between files.
Would this be something worth considering: if you have a file attached and want to ‘pull’ in an object from this file, you send some sort of request to the other user, and if it’s granted, the object is auto-cut/pasted into the active file.
But how would this play together with a ‘real’ server-based approach like in ArchiCAD?