In 1898, five counties became the five boroughs of New York City thanks to an act of the New York Legislature in Albany merging them into one great city. Each borough is a county within New York State.
That 1898 act can be repealed with Manhattan alone choosing its own mayor. The counties of Richmond (Staten Island), Kings (Brooklyn), Queens and Bronx can become independent of Manhattan and choose their own elected officials. It could be done by the legislature and the governor.
It won't happen now, but it's something to work on in the future.