Severus Snape replaced Professor McGonagall as headmaster once Voldemort was controlling the Ministry of Magic through Pius Thicknesse. "Killing" Dumbldedore solidified Snape's value and loyalty to Voldemort. Snape was the Death Eater most familiar with Hogwarts.
In Harry Potter and the Chamber of Secrets, Professor McGonagall replaces Professor Dumbledore after his resignation is forced by Lucius Malfoy. At the end of the year, Dumbledore returns. McGonagall replaces Dumbledore after his death in Harry Potter and the Half-Blood Prince. Severus Snape is appointed headmaster the following year.
After Headmaster SNnpe was killed Minerva McGonagall replaced him. Her term as "true" Headmaster lasted from 1997-2017. After that the only logical answer would be Neville Longbottom who was already on the staff. 19 years later would be 2016 so Neville would be Headmaster in the year 2017. Headmaster Longbottom(2017-
The headmaster or headmistress who will succeed McGonagall after her 2017 retirement has not yet been named by Harry Potter series author JK Rowling. Theories suggest that it may be someone we know from the series. Perhaps Neville Longbottom is in line for the position?
Neville Longbottom I believe. Neville is the head master when Hermiely, Ron, and Harry's children started going to school. I remember the end of the book, one of them mentioning to tell the new headmaster Longbottom Hello.
It isn't specifically revealed but I do believe there was a chat with J.K. Rowling where she stated that Aberforth Dumbledore started to teach after the war and I think she did hint that he eventually because Headmaster.
The board of governors had Professor Snape replace Minerva McGonagall in Harry Potter after Voldemort started controlling the Ministry of Magic in the book The Half Blood Prince. McGonagall had replaced Dumbledore.