you're talking about a world were a princess is the absolute highest level of governmental authority. Title really don't mean diddly doo here.
As long as Equestria is a single nation state there's a lot of leeway, I'm not sure but I think units as low ranked as baronies have been considered independent in the past, Wingbardy or the 12 cities for example.
Celestia and Luna could declare themselves Empresses of course and then add whatever levels of administrative division they felt appropriate between themselves and their territories in order to realize this concept, but they don't want to and have no significant pressure to do so.
Furthermore how are you defining the highest level of governmental authority? Because generally speaking that doesn't exist in a non-specializing Diarchy. Garrick, Rutherford, and Koryu are the Sovereigns of their empires, the sisters are both required to achieve ultimate authority within their domain, at least for now.
Speaking of Luna's future issues:
I wonder if she might do better roaming farther afield? Perhaps overseeing contact with the various other nations, or helping to arrange a larger diplomatic set up to allow for the various nations to more easily communicate with each other. Essentially she would be Equestria's field princess.
Yes it's inspired by the game faction omake where Luna was basically a roaming horde faction. I'm just translating that into a post unification wars setting.
Now normally we don't have that kind of control over the decisions of a foreign government, but I think we could nudge her in that direction with various projects, especially if we ran our plans for thestral immigration by her and just generally made her our point of contact.
