Disclaimer: All plots and characters you recognize belong to J.K. Rowling.
This story will only loosely follow the events of HBP and the succeeding books.
Her Brilliant Mind
She stood in front of the gates, transfixed at the sight before her. The castle, so tall, so imposing, made her feel like an 11-year-old girl again. It seemed as if no Battle of Hogwarts took place more than a decade ago. She was a bit breathless from having apparated from Hogsmeade, and she clutched her sensible leather satchel, a facsimile of the beaded purse she had taken many years ago while on the run with Harry and Ron. The rattling sound of books made her smile wryly as she took several breaths to steady herself.
Might as well get this over and done with, she thought. She raised her wand to send out her Patronus when she saw two people hurrying down the path, matching smiles gracing their faces. One of them waved the gates open, allowing her a wider view of the Hogwarts grounds
"Hermione, our dear girl." Albus Dumbledore beamed at her as Minerva McGonagall enclosed her favorite cub in a warm embrace.
"Professor Dumbledore, Professor McGonagall." Hermione smiled at each one in turn before addressing the headmaster. "I trust that you are both enjoying a quiet vacation?"
"Oh, you know, it's the same every year, Hermione. And it's Albus and Minerva now," her former Transfiguration teacher gently chided her.
"We'll catch up before the faculty meeting tonight," Albus said cheerfully as they started walking along the path headed to the front entrance of the castle, "but I really must say that we appreciate you considering our request. You see, my dear, there is no one else who can fill this position."
Hermione gave a non-committal shrug as she hoisted her satchel over one shoulder. "I quite understand…Albus," she said, struggling over calling her former teachers by their first names, "but I think it would be better if I let you know as early as now that I have no intention of staying for more than one school year. I think that's all it would take to complete the work at hand."
"We'll see," Minerva said, her benign statement belied by the hint of mischief dancing in her eyes. "Now, let's get you settled in, shall we?"
Hermione nodded in assent and allowed herself to be ushered along the path, her eyes wandering over the vast expanse of the school grounds. All around her, flowers were in full bloom and the mingled scent of sunlight and magic hung in the air. The gentle breeze rustled the leaves of familiar trees and she could make out several owls soaring overhead, heading towards the castle.
Hermione was suddenly seized with panic, an unwillingness to enter the castle she had left as a girl. I don't belong here anymore, she thought in despair.
As if reading her mind, Minerva allowed Albus to enter the castle before them and faced the former Head Girl. "Now, Hermione," she began, "Albus and I are fully aware that what we are asking of you will surely slow down all your other projects. We debated at length before we even ventured to owl you that first letter asking you to come back."
"I am aware of that...Minerva," Hermione said softly, gazing at her former teacher steadily. "And I am happy to come back and give back to the school that has given me so much."
"Still…I do not wish for this project to be a source of discomfort to you," Minerva said, brows drawn together in consternation.
"And why would it be?" Hermione parried, her expression only conveying vague interest.
Minerva remained silent for a while, looking at her appraisingly. Hermione inspected the castle doors, her mind already wandering to magical algorithms.
"Just know that should you have any hesitations," Minerva reasoned, "any at all, then we can wait for a few more years, or look for someone else who might qualify. We will not think ill of you, my dear. Far from it."
Hermione let out a short bark of laughter. "But there is no one else, Minerva," she said matter-of-factly. "I've made the calculations. This castle will cease to exist if we wait longer. Whatever needs to be done needs to be figured out and done now."
Minerva sighed.
Both women looked at the castle. The castle that had shielded them during the battle stood regal and imposing, but was decaying from the inside. Too many spells from too many wizards and witches belonging to houses that should have been united had skewed the magical architecture harnessed and set by the founders. The castle was now on the verge of collapsing. A time-released document unearthed by Albus had clearly stated that in the event of the castle's collapse, Hogwarts and everything non-living housed inside will vanish, to become unplottable to both magical and non-magical folk, in a last-ditch effort to safeguard its magical secrets.
"If you're sure, then," Minerva ventured.
"I am," Hermione said.
"All right, then," Minerva said, turning kind eyes on her former student. They grasped one door handle each and swept inside the castle, together.