I would really like a way to add a collapsible fleet grouping that can contain multiple fleets under a single NAC (without having to nest another NAC).
It can work in the UI perhaps exactly like a NAC does.
Currently, I have so many fleets in a single NAC that some fleets in the middle of the list can't be dragged to another NAC, because no other NAC is visible in the pane at the same time as these fleets. The workaround for that is rather tedious: create a new empty fleet, move it to the same location (which often requires creating new waypoint), and give my existing fleet an order to merge with the new fleet.
A fleet grouping node would also be useful for general organization, like grouping my Logistics NAC by general logistics type (slow mineral haulers, quick installation movers, and population movers), or grouping my Survey NAC by galactic region.