Harry was headmaster fic
I dont remember too much of this, or if it was complete d or abandoned.
But Harry and Hermione were brought through time to a future where hogwarts was closed.
They found out a first year that was brought into the Wizarding world had been a famous genius in the muggle world and ic redonly gifted student. They realized magic was the other side of the equation with physics/quantum physics in muggle science. His breakthroughs revealed the magic world.
He was the last headmaster of Hogwarts before the muggle governments closed it and now any magical kids are tsken to special government magic education centers.
That was what I remember of it.