The Last Couple on Earth
The problem with Time Turners was that whatever time travel occurred in the future had already been carried out in the past. You could go back with Time Turners, but only because your future self had already done so, so while the fact that you would have a Time Turner in a little while would change the present compared to if you wouldn't have Time Turner in a little while, that was only because your future self had already gone back and done what your present self was going to do when enough time had passed for your present self to turn into that future self.
A little confusing, but after a lot of time thinking about it and listening to Hermione explain it again and again, Harry got it. Even the particulars.
The upshot was that you couldn't use a Time Turner to go back and undo Armageddon because if you could, your future self would've already done it, but Armageddon had happened, which meant you hadn't stopped it, which meant you couldn't.
A large section of Harry's brain had become devoted to time loop diagrams and flow charts, all to no avail.
They considered sending just their souls back, but that too came up against fundamental restrictions. The universe just didn't like information coming from the future into the past. Even Seers were only looking at the machinations of destiny, not the future.
Then Harry found a loophole. He was rather proud of that, even if it was Hermione who did the majority of the intellectual legwork worked needed to make the idea into a magical reality. It turned out, the universe didn't mind the future telling the past what the past already knew. Maybe you couldn't send a soul to the past, or, for example, tell your past self the location of Voldemort's horcuxes, but you could send a message consisting of nothing but common knowledge, common sense, and general wisdom.
Hermione especially felt queasy about breaking the timeline, but it ought to be restricted, spatially speaking, and it wasn't as if there were many people left alive on earth to be killed by it. Voldemort's war had been about the worst circumstance imaginable for the Statute of Secrecy to come down in, and the rest was history, Imperios and bombs.
So the question came, at what particular point in history could a message of bland platitudes avert Armageddon? They'd moved into a temporally displaced pocket dimension to avoid all the radiation, and they thought that as a result they might just survive the timeline breaking, but even at best, they'd have a limited number of shots at it. Probably just one.
They considered trying to impact the course of a young Tom Riddle's life, but sending a message back to the point before which you had done any magic was, if not fundamentally impossible, very, very difficult, and besides, Harry thought that Voldemort had been born a bastard and there was nothing anyone could've done about it.
It was easiest to target yourself. Nothing had better sympathetic properties with you than yourself. So they poured back over their own lives, not just viewing their memories, but viewing them in the past itself, as if they were angels looking down from heaven. As much as it pained Harry to admit it (and as much as Hermione assured him that, considering his upbringing, he'd done fantastically), the world had turned on the fact that Harry had become a bit of a lazy prat in his teen years.
Further, the one who'd led him into that was Ronald (may he rest in peace) Weasley.
The obvious solution was to stop Harry from becoming friends with Ron. Perhaps they could manipulate events such that Harry ended up sitting with Hermione and Neville on the way to Hogwarts.
Except they both loved the bloke and he'd come in handy fairly often, and, before everything had gone completely to shit, they'd seen glimpses of a motivated, engaged Ron who wasn't controlled by his own insecurities, and he'd been a man to be reckoned with.
So they'd gone back over their lives and focused their attention on Ron.
The Ron from first-year was a bit of a prat, but while he wasn't a great student, he hadn't been bad either. He'd done his homework, studied occasionally on his own initiative, spent serious time in the library looking for mention of Nicolas Flamel, and had been known to pick up a book, academic and otherwise, just because he wanted something to read.
Heck, his favorite hobby had been chess, and he'd given regular beat downs to upper-year students who fancied themselves good players. That bespoke a lot of promise that had never been realized except in further chess dominance.
Instead, by his third-year, Ron had become a committed anti-intellectual making an art form of slacking.
It hadn't taken long to see from whence the change had come.
Ron had spent the entirety of his second year with a broken wand. Unable to cast spells. On an obvious level, that meant Ron had lost a year of practice at casting spells. But it worse than that. Coming to class and your wand not working was literally a common nightmare Hogwarts students had, and Ron had lived it for a year. His lazy attitude, his proud ignorance, his mocking of learning, had all been defence mechanisms he'd developed to protect himself from daily mortification. His contempt for success had been the result of enduring a year in which success had been impossible.
When Harry had realized that, he'd raged at himself for days. What a horrible friend he'd been. He should've gotten Ron a new wand as an early Christmas present within weeks of the start of term. Instead, he'd ignored it, and thought Ron's jokes about it funny while Ron had spent every day in a constant state of intense embarrassment.
Harry channeled that anger into work.
A ghostly projection of a great wizard stood on a side road, awaiting what he theorized was, in an odd way, the most important moment in world history. He seemed to be leaning against the trunk of a large oak tree near an old turquoise Ford Anglia, though a closer inspection would reveal that he had actually passed a few inches into the bark of the oak.
Fortunately, muggles couldn't see him at all.
Two boys approached the turquoise car, one black-haired, one red-haired, both pushing trunks, the black-haired one with an owl on his trolley. They didn't notice him, because he was, after all, hard to notice, even for wizards. The red-head unlocked the trunk with a series of taps from his wand, and they heaved their luggage in.
The black-haired boy was about to put his owl on the back seat when the projection spoke. "That's a very dumb idea," it said.
The boys whirled, and they draw back when they saw him, eyes narrowing, hands reaching for wands. He knew about what they saw. Green eyes, black hair, and an indistinct face. Oh, it was all there. The nose, the mouth, the chin. They just wouldn't come together.
The black-haired one said, "Are you a ghost?"
"Call me Harrison," said the projection. "And you're Mr. Potter and you're Mr. Weasley, and what the two of you are planning is very foolish."
Ron said, "What we're doing isn't any of your business."
Harrison smiled and spoke. His speech came out garbled, completely incomprehensible, lexical white noise. Both boys frowned at him, and Harrison sighed. He had tried to say, 'Tell Dumbledore that Ravenclaw's Diadem is a horcrux and it's in the Room of Lost Things, which he may find out about from any Hogwarts house-elf.' But, as expected, it hadn't worked. Onto the plan, then. Harrison said, "Tell me. Do either of you play chess?"
Both boys nodded cautiously.
Harrison said, "Would you ever push a bishop to a space without thinking about what would happen? Without even checking to see whether it was protected? Without considering about what the result would be two or three moves later?"
"Of course not," said Ronald Weasley, while Harry shook his head slightly, though Harrison knew that Harry often still failed to think even one move ahead.
"If you're not so reckless with your chess pieces, why are you so reckless with your actions? In life, there are times when you must react, times when you must depend on instinct and intuition, but generally, you should consider the consequences. And it is when the stakes are highest that you must consider the consequences most carefully. Mr. Weasley, what does your father do for a living?"
"He's the head of the Department for the Misuse of Muggle Artifacts," Ron said, sounding proud. The head of a department, after all.
Harrison said, "What do you think will happen to him when his son is caught misusing a muggle artifact? One that he himself enchanted?"
Sounding shocked, Ron said, "We wouldn't be caught. There's an invisibility booster."
"And if it malfunctioned? Here? In the middle of London in broad daylight?"
Ron paled. "The Statue of Secrecy… Da' could be fired."
"And even if worked, what do you think would happen?"
Harrison watched the gears turn in the boy's head. However much he might've seemed it, Ronald Weasley was a long way from stupid. He was realizing that he'd be in awful trouble with his parents, and that the school would find out and he'd lose points for Gryffindor.
But though Ronald Weasley might be capable of sound reasoning, he'd seldom been one to let it change his mind.
Ron said, "Fred and George did it."
"They had a moderately strong reason, they knew what they were doing, they did it at night, and even then, it may have been a mistake. You just want an excuse to fly the car. And Scotland is a lot farther than Surrey. Imagine if the car failed on the way. While the two of you were in mid-air."
"How do you know about all this?" said Harry.
Harrison smiled again, and his eyes twinkled. "I wish I could tell you. I can't. I think you'll find that strange old men sometimes have secrets concerning you which they keep from you. Maybe they shouldn't. Still, that doesn't mean you should keep secrets from them." He leaned in and said, "If you hear a voice that no one else can hear, tell Dumbledore."
"What?" said Harry, and Harrison knew that on that last sentence, his voice had come out garbled.
Harrison said, "Go to adults for help. It won't always work, but when it does, it will be wonderful. And don't assume they know everything, because they don't, no matter how all-knowing they seem. When there's a mystery, the clue that's strange and meaningless to you may be the missing key for someone else." There. Hopefully when Harry heard the damn basilisk in the walls, he'd tell Dumbledore about it. That would make his second year a lot less traumatic.
A pair of voices grew closer.
"Honestly Arthur, if we'd just floo directly to the platform like most people, we wouldn't be in such a rush."
"It's important that they see the muggle world. And going to through Kings Cross is traditional."
Harrison smiled. His preaching likely enough hadn't done any good, but he'd delayed them long enough, and that was what was important.
Still, Harry's parselmouth ability would come out eventually after all, so he had one more platitude to give. "Don't ever be ashamed of being different," he said, and disappeared. As he faded, he heard Mr. and Mrs. Weasley asking the two boys why they weren't on the train.
Harry Potter stepped out of the machine, and Hermione Granger took his hand.
"Well?" she said.
"It went how we thought," Harry said. "No way to know if it really mattered. What do the instruments show?"
"The timeline is breaking all the way up the stem. About twenty seconds until it hits the present. No indications of whether our pocket dimension will survive."
There was nothing to say to that. It was what they'd expected going in.
They sank on the floor, holding each other tight, Harry trying to be aware of nothing but the feel of her. Perhaps it was selfish, but his last thought before the timeline was to hope that the little Hermione out there in the altered reality fall in love with the Harry there without Harry having to be the last man on earth.
This could've been rather good if I'd been willing to spend 10 thousand words on it, but I wasn't. Ah well. It's written.
Seriously. I know Ron only needed his wand for Transfiguration or Charms, but I have to think that Ron's second year is low-key the second worst year any of the trio had, after only Harry's 5th.
tbc, this is a oneshot. I may feel like adding onto it eventually, but for now, it's done.