Hi Martin, thanks for reaching out. I think you're pretty spot on with your assessment of the requested idea.
An example on the electrical side is when the equipment is in a linked model and needs to be circuited in the host model. The goal is to create a circuit with the link's equipment's connectors and then wire the circuit in the host model.
Continuing with that example, the same would go for duct and piping systems. If the equipment was in a link, it would be nice to be able to create a system in the host model and then connect duct/pipe to the connectors on the equipment in the link. Then as you stated, the flow should propagate through the system, back to the fan/pump/equipment.
One thing I would add is that it would be helpful to have the circuits and systems be visible in both/all models. So if the mechanical equipment is in one model and then there is an electrical model that contains all the circuits, I think it would be nice to have the circuit information be available in the model with the mechanical equipment. I'm not sure how this would be handled (it's still an idea!), but I'm thinking the mechanical model would also have to link the electrical model for this to be able to happen. In theory, the mechanical model could be linked to several electrical models with varying circuits/panels, so there would have to be a means to communicate the correct information back.
In the case of extremely large projects with multiple models, it is important to have a method to keep track of all the system/circuit information.