I'd like to see top-tier administrative divisions mapped across provinces, somehow. This would also enable Thirteen Colonies to be governed as 13 "sub"-colonies while provinces would essentially serve as its counties. One thing to keep in mind is that administrative divisions aren't all same. Counties in the England was kind of an equivalent of the states in the U.S. except that it doesn't have sovereignty, has Crown appointed officers like Sheriff and Justice of the Peace, and generally do not have legislative body (afaik) but nevertheless there is nothing above it except the kingdom. That being said, I would suggest some kind of custom name schemes to go with administrative divisions.
Administrative divisions can also act differently depending of forms of government. Federal republic would cause divisions to act more independently than it would be under constitutional monarchy. Alternatively, make federation or unitary state a separate component to be set upon change of a government. Think of it as a form of subgovernment or something. Feudal monarchy would almost always start with unitary state, tempered by autonomy and estates so it is not totally centralized and powerful in the beginning. It's also possible that federations would probably only make sense if there were some sort of dividing lines between regions or something.
For example, confederations (much weaker than federations) might be more strongly desirable if all states within a country adheres to different religions, are of different cultures, but have common interests in defence and economy. The prime example of this would be Switzerland. Switzerland in fact speaks more than one languages including Italian, German, and French, and practices Protestantism, Reformed, and Catholicism.
Federations may also be desirable in diplomacy if one seeks to expand a state peacefully as much as possible (though not totally refraining from using force) and setting your government to federation might make some countries more willing to join as vassal or join the country. Good example of this of the federal German Empire that actually had not only the emperor (kaiser) but also kings of states like Bavaria that voluntarily joined the preceding confederations and the empire, while Prussia that created the empire also conquered other states that were unwilling such as Hanover (the former partner of personal union with Britain).
Also, administrative divisions does not need to be more than two provinces but mechanics would need to be made in a way that it is strongly preferred to give administrative divisions more than one provinces. There are couple ways to resolve this. One way is by fixing the administrative divisions to de jure boundaries like in the Crusader Kings II so you can only assign a province to its de-jure division. Another way is to create the limit for how many administrative divisions you can have in a country, scaled with how many provinces you can have.
