National Instruments will not be implementing this idea. The LabVIEW Project Window can be used for logical file organization, and the top-level VI(s) of the project should be easily identified by their location in the project hierarchy.
LLBs always had the capability of marking a VI as toplevel.
I wonder if it would be possible to set a flag inside a VI so a VI is recognized as such from within e.g. windows explorer. (e.g. different default icon). Since this requires OS integrations, I don't know if this is even feasible.
Still, It should at least integrate with the LabVIEW project.
The default settings could be very simple:
A VI without any defined connectors in the connector pane is "toplevel".
If there is at least one defined connector, the toplevel designation is lost.
There should be a way to overwrite this in VI properties.
Many times, people attach a zip full of VIs without telling us the name of the toplevel VI, so this idea would limit the number of candidates to inspect.