iTwins has listed an impressive number of external formats. But I understand that this is only one-way 'synchronisation' only, ie importing from the format to itwin.. not like the ISM Structural Synchronizer workflow which supported bi-directional 'update from repostiory' mode. ISM seems to be moving to iTwins Analytical Services.
Even with dgns, this is only possible using PlantSight?
It would be good to provide a bi-directional Connector for IFC and Revit too, start with. Mayb IFC initially since there is an exporter in place.
If iTwins is to function as a hub it needs bi-directional ISM-style synching to be useful. The Issue Resolution Service route is a roundabout and inefficient way of propagating changes and will suffer the same problems as mainstream clash detection workflows which rely on electronic Post-Its to get changes done asynchronously. Clients and app users have long realised it is bettter to mandate a single platform so that you can make the changes in real time without conversion lag; and ideally in the same model. The Revit ISM workflow is good case in point, here. You don't write a BCF or redline IRS item telling another to change the beams in another model. You do it within your app and sync the changes across.
iTwins can be the hub for this and other workflows.. but it needs to support bi-directional synchs.
The business data 'I in the BIM' side of things is probably easy. PlantSIght's EDM is probably the starting point. I can see a lot of users using iTwins services to add, modify, delete, format info to the attached to published info. As workflows become more data-centric, iTwin's java/type-script should help users copy and paste from a much larger coding ecosystem compared to the old VBA, dotNET, MDL CAD world which seems to stagnating.... to the point I can see a lot of future Mstn drivers wanting to automate things using iTwins and synch back to dgn (or any other CAD format) after they are done.
Again, iTwins can be the hub for this and other workflows.. but it needs to support bi-directional synchs. Ref attaching iTwins is a good start but should not be the end.