I do not own any of these characters. They all belong to JK Rowling.
When the Battle of Hogwarts was ended, Minerva approached Harry. "Could you come with me for a moment?" she asked him. Harry nodded his head and followed his Professor McGonagall to her office. There was some rubble scattered around the room from the destruction. Professor McGonagall went to her desk and pulled out a sheet of paper.
"I thought you would like to have this. I got an Owl years ago, and it was a letter from your father," she said. She felt tears prick in her eyes, thinking about the letter. It was the last letter that she received from James and Lily because two months later, they were brutally murdered by Lord Voldemort. Harry opened it up. It was a picture of him on a broomstick and chasing a ball that his mother was charming. Harry turned it over and read:
Dear Minnie,
You know how you never thought that you would have an amazing Quidditch player on the Gryffindor team, I am proud to announce that you are wrong! Lily and I made you the next best thing. Harry here, is going to be even better than me I think! I didn't even think that would be possible! I hope you are doing well. We all miss you, and Sirius is still waiting to hear from his proposal. Lily sends you her love.
Your favorite students,
Sirius, Lily, Remus, and James.
Harry smiled reading the letter. He was torn from taking it home with him, or let Minerva keep it. He decided in the end to let Minerva keep it. It was meant for her. He passed it back to her. "Professor, my dad wrote this for you, I think you should keep this."
"The day that I saw you in the first year, catching the Rememberall, I knew that he was right of course. And if you repeat this, I will deny it til the day I die, you were my favorite student, even though you got into so much trouble with Hermione and Ron."
"My children will be worse. I plan on marrying Ginny." The color ran away from her face. She could not even fathom the thought of a Potter and a Weasley child. They were going to be worse than Harry, Fred, and George put together! Harry could not help but laugh at Professor McGonagall's reaction. Before Harry or Minerva could speak, there was a loud crash outside her office.
"Peeves!" she huffed in frustration. She opened the door but instead of seeing Peeves, she smiled, seeing the spirits of James, Lily, Fred, Sirius, and Remus. She couldn't help but laugh, glad that they had adopted Fred into their group.
"Hiya Harry! Why didn't you tell us that your father, godfather, and Lupin were Maurauders? You sneaky little bugger!" Minerva laughed with joy, shaking her head. Tears came into Harry's eyes, seeing Remus and Sirius. All he wanted was to give them both a hug, thanking them for raising him.
"Harry!" came three voices, "Where are you?"
"Up here!" he shouted back.
Ron, Hermione, and Ginny came running up the stairs to meet him. "Fred!" Ron gasped.
"Hiya brother, you okay?"
"Ya, but how is this possible?"
"When I passed over, Prongs, Padfoot, and Moony met me and we decided to pull pranks on Hogwarts, so I will forever be here, causing havoc with my new friends," he answered his brother with a smile.
"Harry, Hermione, Ron, I cannot tell you how much you remind us of ourselves. Cherish life for as long as you can, because you never know when it will end. And Minnie, I am still waiting for an answer from you about the proposal," Sirius joked, looking at Minerva. Minerva could not help herself and laughed. Despite the ever growing headache that she felt was coming on, Minerva was happy.
After the school was repaired, and there were many funerals, they decided to keep the school open. Minerva became headmistress of the school and after a few years of being an Auror, Minerva welcomed Harry with open arms to become the professor of Defense Against the Dark Arts. Just like they had promsied, the Maurderers reeked havoc in the school. She did not know who was worse, Peeves, or them. The students that did not have any place to go to during the summer, was able to stay. Harry was so proud when his godson, Teddy Lupin, got into Hogwarts. He had no doubt in his mind that he was going to get in. He was put in Hufflepuff, so he was able to see his mother every day. Harry was even more prouder of when his children got into Hogwarts.
As a surprise to everyone, Dudley's daughter was accepted into Hogwarts. Harry promised Dudley that he would keep an eye on her while she was at school. Over the years, they were able to repair their relationship. They were not best friends, but they were friends. Harry taught Dudley everything that he needed to know to help his daughter when she came home over breaks. Dudley even visited the school. Harry got special permission from Headmistress McGonagoll. They had opened the school to all Muggle parents and siblings.
When Minerva retired, the day that Harry's youngest son arrived at Hogwarts, she offered the position of Headmaster to Harry. She could not think of anybody who deserved it more than him. He had worked so hard to become successful, and was a wonderful teacher. Harry was more than happy to take the position from her. Minerva stayed at Hogwarts even when she was retired as headmistress and helped out whenever she could. She enjoyed seeing the children grow up. Everybody was taken aback when Minerva helped Sirius, James, Lily, and Remus pull a very large prank on Harry. She enjoyed her retirement. Harry was happy that she decided to stay on.