America in city states: States of America.
If it has to be changed from United States, I'd suggest something like Perpetual Union (of America?), which calls back to the Articles of Confederation, a much more city-statesy arrangement than the Constitution.
City States represents literally independent cities, not any loose political association of any kind. It's an anachronistic civic in the late game.
In general yes, the question is, do we let Hitler decide what you can make part of your empire in the game? It all boils down to the question of what vassal states actually represent, and that varies.
Hitler was a great admirer of Frederick the Great. Surely he knew about the Anglo-Prussian alliance. I believe that he genuinely wanted to avoid a war against Great Britain, at least until Russia was subdued. But Churchill would not allow that.England was never intended to be part of 'The Greater German Reich' - Hitler used the term 'The Anglo-Saxon Kingdom' and wanted her to be a 'great naval ally'.
He intended to compromise with Arab governments and wished to establish 'Reichs-protektorats' in some areas of the Arab world.
The three weirdest vassal names I've seen were:
The Portuguese Continuing Revolutionary Proccess (I have no idea what caused this...)
The Portuguese Government in Exile (once again, I couldn't tell why)
The Latin Empire (Byzantium, when vassal of Holy Rome)
What caused the first two? And should a vassal be called an 'empire?'
The three weirdest vassal names I've seen were:
The Portuguese Continuing Revolutionary Proccess (I have no idea what caused this...)
...
I saw this Name once, IIRC its their Name, when they´re Communist and not vassal of Russia.