Really, there's nothing preventing any existing state from adopting a federal structure internally. All it means is that the state is made up of organic constituents that are not alterable beyond certain narrow limits by the state's government.
Cities and counties are all creatures of the state government, and can be modified without their consent by the state government. A federal state would simply be made up of republics that were presumably larger and more influential than a county would be, and have their own elected governments, administrations and constitutions.