50 years later.
Harry and Hermione stand casually in front of Hogwarts' main gate. The school was familiar and yet different. A great dome had been built over the school to protect from the out of control pollution, and allow students to roam outside without a bubblehead charm.
Unknown to most citizens of the magical world, the new and very eccentric headmaster, Luna Lovegood, had allowed the two founders of magical flight unfettered access to Hogwarts' grounds and today was the day they would unveil their work. While it was supposed to be secret, most of the magical world knew something was happening and a large portion of the British and some western European magicals had gathered in Hogsmeade, under the smaller village's dome.
"Good morning." Luna says casually as she arrives on her Thestral drawn throne. The years had been unnaturally kind to her and she could still pass as an older university student if she wanted to, unlike the two Potter-Granger's whose many hours in a workshop, or traveling left them with weathered skin and somewhat wild hair. They could only pass as a couple of mid-thirties working people at best.
"Good Morning Luna." Hermione says promptly, "I can see you are excited."
"I am even wearing the coveralls you gifted me when I first started learning." Luna says dreamily as she glides off her throne with the grace of long practice.
"We can tell." Harry says with a chuckle, Luna barely fit in those coveralls, and no one would believe she was the headmistress of a school if they saw her. Not that Harry or Hermione were any more appropriately dressed. Scottish summer wasn't what it used to be and the sweltering heat was more like what London would have experienced half a century ago. Harry had his overalls half off, with nothing but a stained sleeveless shirt covering his upper half, while Hermione had chosen to remove her shirt and only the overalls straps protecting her modesty.
This was rather common these days, with the heat reaching deadly levels for months on end in many countries, dress norms had adapted. Even with non-magical folk wearing gas masks while outdoors.
"Anyway, we might as well start." Hermione says, stepping through the main gate and singing about the long walk to the attic of Luna's office.
"What are you doing?" Luna asks, causing Hermione to stop.
"Walking to your office?" Hermione responds with some confusion.
"Don't be silly," Luna begins while reaching for Hermione's arm, "It's much fast-"
The world distorts as Luna grabs Hermione's arm mid sentence and apparates the trio to her office.
"-er to apparate." Luna finishes before drifting off as if nothing strange had happened.
"Luna!" Hermione shouts while Harry struggles to hide his giggles, he knew Luna well enough and had offered his arm in advance.
After letting Hermione cool down the three walk to one of the bookshelves and Luna pulls on a rather dusty tome. With a click the bookshelf begins to rise, taking the three with it. They arrive in a very modern looking room, with consoles and paper thin communication mirrors. The trio casually move towards the three comfortable chairs on a slight pedestal. Luna sitting in the center with Harry on her left. As they sit, screens pop into reality in front of them, displaying all sorts of information. Hermione doesn't hesitate and is instantly typing away. Lights of all sorts go green on the lesser consoles around the room and soon a large holographic green button appears on Luna's console.
"All systems ready." Hermione says.
"I am ready. Test begins on your mark." Harry adds, looking at Luna.
"Mark." Luna says with unusual concentration as she hits the green button.
The Village of Hogsmead was like a festival, witches and wizards of all ages were having a massive picnic. With free butter beer for all, courtesy of the now quiet fat Ronald Weasley. He and his equally heavy, dark skinned American wife were at the center of it all, sitting on two chairs set upon one of the Three Broomsticks' sturdier tables.
The feasting and cheering came to an abrupt stop when the grand castle of Hogwarts rose into the air.
"Well, I didn't expect that." Fred and George say in unison, as several magicals faint in shock.
As Hogwarts rises to a comfortable altitude, the enormous effort of Harry and Hermione is revealed. They had, without anyone noticing, extended the dome five stories underground and then added an assortment of hangars and runways. Below this was a flat floor covered in thrusters of all sorts. Massive props extended and tilted to provide directional control as more modern air rockets, which were magic jets without an intake provided the majority of the lift. Luna's influence was clear here as no single thruster was identical and some had the most unusual of shapes. This somehow provided a perfect balance of lift so the more educated onlookers assumed it was calculated.
"Flight seems stable." Harry reports as Hogwarts settles at a kilometer and a half above hogsmeade.
"Magic use is in balance, systems charging and discharging properly." Hermione reports.
"Then on to test two." Luna commands.
With this statement, Harry presses a few buttons and a holographic set of pedals and an additional joystick appears. They had designed Hogwarts with the same controls as a helicopter, even if it handled more like a large blimp.
"Wait, Dobby needs to land the Great Griffin in the main hangar first." Hermione says urgently.
After a few moments, Dobby manages a somewhat soft landing of the giant aircraft. Hermione only winces once as she watches through one of the mirrors.
"Now we can go." Harry says impatiently, tilting Hogwarts slightly and increasing throttle.
"Magic levels are still stable." Hermione says as she scans dozens of messages on her console.
Luna also begins working and helps keep Hogwarts relatively level with delicate adjustments to the many thrusters.
"Flight test confirmed." Harry says as Hogwarts reaches thirty knots.
"Time for test three!" Luna says with growing excitement. Harry nods wordlessly and with a mighty groan Hogwarts begins rising, and fast.
"Magic levels falling as expected." Hermione says stoically. She was too focused to display emotion at this point. Her two companions were far less reserved and could barely focus as they were overwhelmed by joy.
To the utter shock of everyone in Hogsmeade, Hogwarts disappeared into the blue sky.
"Well, we aren't hiding this." Harry says as Hogwarts settles into orbit.
"If anyone still doesn't believe in magic they will now." Hermione comments as her eyes race through thousands of readouts.
"I think we deserve a cup of tea before we do the last test." Luna says and tea appears on a floating tray in front of them.
"Indeed, we can let the non-magical media have a few pictures." Harry says, grabbing some tea and relaxing.
"I need more than tea to relax." Hermione says before downing her tea in one gulp and standing rapidly.
A few hours later.
The three settle into their chairs and look through the data from the last few hours.
"Looks like magic levels are refilled properly." Hermione reports.
"Minimal orbit decay, automatic pilot is close to perfect. We can calibrate it easily." Harry reports.
"Then onwards to test four!" Luna shouts.
At this point the people of hogsmeade were rather drunk and quite full. The twins had managed to set up a few full sized mirrors connected to a couple of telescopes, allowing for some view of Hogwarts as the sun sets.
That was until Hogwarts disappeared in a blur.
"Did they just portkey Hogwarts?" Fred asks.
"They did." George replies.
In only a few seconds the view from Hogwarts' bridge changes from Earth to the Moon .
"Test four complete." Luna says.
"It's beautiful." Hermione says, gazing at the moon's majesty.
"We've been here before." Harry says.
"Not the moon, the success of our life's work!" Hermione says with a tear in her eye.
"Who would think a simple protest of school rules would lead to this." Luna says seriously before adding dreamily, "Maybe I should make more rules, then we can get even more innovation."
"Of course Luna." Harry says. "If only Nigel could see me now."
To this day Harry had fond memories of his flight instructor, who had died in his sleep many years ago. Most believe he died peacefully, but only his late, and now passed wife knew he died during his recurring nightmare of that blasted biplane that got away.
AN: And there it is, the end I've been thinking about for a long time. How they got to this point, and what will happen now is up to your imaginations. Part of me wanted Harry to do a barrel roll with Hogwarts, but he's grown up now, if only a bit.
I've had a lot of fun writing this, and while it's not perfect it is my contribution to this massive fandom that I've spent so long reading. I do hope to find some new plot-point for Harry Potter, but don't bet on it.