Saving James (with time travel)
The time-traveler looked at James with large grey eyes. "Remind me again why you need an unbreakable vow?" He asked her.
"The future can't be changed," she explained, "believe me I've tried." Not very hard if she were honest, too well aware of the theory and what could happen to her if she pushed. She'd tried only enough to convince herself that the accident that had send her into the past probably still enforced the known rules of time-travel. No need to tell James of her doubts, she thought. Easier this way.
"If I tried to tell you what you need to know without an unbreakable vow, I'd probably just get a stroke and die."
"Alright, alright, I'll give you your unbreakable vow," James acceded. "Who will be our bonder?"
"Lily would be ideal," the time-traveler said. Looking at her watch she added "but we don't have much time. I've only got 45 minutes left."
"This had better not be a joke" Lily fumed, as James dragged her towards the time-traveler by her wrist.
"This," James announced getting to the time-traveler, "is a time-traveler."
"Please" Lily groaned, rolling her eyes. "You said it was not a prank. Proof it."
"33 minutes," the time-traveler said.
"Please please please please just bond our unbreakable vow for use?" James begged.
Lily huffed but placed her wand over the intertwined wrists of James and the time-traveler, and James began his vow.
"I James Potter vow, so long as this missus' predictions about the future proof true, not to willingly change the future to make it different from how it is revealed to be and not to tell others about the future as it is revealed to me, except to Lily and to only tell her if she takes the same vow.
The vow took, James felt a surge of magic travel up his arm, and Lily looked at him in shock. She realised at that moment that this was not, in fact, a prank.
"Please leave us?" James begged.
"Let me get this straight," James said. Inside he was fuming. "I will marry Lily and we'll have a child, and come Halloween 1981 the whole wizarding world will think me and Lily are dead and there's nothing I can do about it?"
The time-traveler nodded. "And they must keep thinking you are dead until 31 July 1999 at the earliest," she added.
"And our son…?"
"He'll have a tough life, but he'll survive. At least until 31 July 1999."
"Why did you tell me?" James asked despondently, "What do I gain from knowing?"
"If I had not told you, or if you do not tell Lily at the opportune time," the time-traveler explained, "Lily will safe the life of your child and reduce Voldemort to a wraith-like state with a ritual. The ritual is theorised to be based in blood-magic, though I suspect soul-magic could also be involved. If I had not told you, or if you do not tell Lily at the opportune time, this ritual will likely require the sacrifice of a freely given life, and you'll likely be kept in the dark about it. If you do tell Lily when the time is right, you can work on it together and you will have the advantage of knowing that you will succeed. Look through your family grimoires, your dark, hidden and cursed books. I'd recommend also researching the rune that's shaped like a lightning bolt. Your trap must not only rip Voldemort's spirit from his body, but also produce credible fake corpses for yourself and Lily."
"You sure?" James whispered, his head held in his hands, "this sounds hopeless."
"Remember," the time-traveler chirped happily, "from my perspective you've already done it. I have complete faith in you." She beamed at him.
"Any more advice?" James asked.
"First, if you want Lily to love you, you should try being nice to Severus. Second, Voldemort is probably destabilised by the creation of a large number of Horcruxes. Third, I heard Australia is nice, but if you go to New Zeeland you might find all kinds of exciting creatures. And Fourth, um… Frank and Alice should build a bunker in their minds to hide from torture. They might have a bunch of cruciatus coming for them. Without such a mental bunker they'd probably go insane and they might even spill all their secrets. I'd recommend a bunker which they can only be retrieved from with the password Nargles. N. A. R. G. L. E. S. Nargles."
The time-traveler comforted James for a little while longer, and told him a little about his future child to give him courage and strength. And when her time was almost up she announced she'd always wanted to kiss James Potter, and, with a mischievous smile, kissed him on the tip of the nose before fading from view.
James verified that the time-traveler not only knew a lot about the Marauders, but also correctly predicted the birthdate, gender and name of the children of several of his friends. On his wedding night he brought Lily in on the secret and they continued his research together.
"I think I won our game of hide and seek!" Luna exclaimed in the frantic living room, generating an even more chaotic response.
"Thank goodness there you are"
"What were you thinking"
"I think we got out-pranked brother-of-mine"
"Are you all right?"
When everything calmed down, she looked straight at harry, and said: "please don't be mad. I got you the best birthday present I could. Your parents should be here before the evening is over."
Harry looked at her dumbfounded. "Why would I be mad?"
"I know right? What's a little necromancy among friends?" Luna asked with a bright smile.
Chaos erupted again.
The doorbell rang, and a very excited Hermione pulled two strangers inside. "She did it! She did it!" she exclaimed as she dragged them through the house.
Harry smiled like the sun. "I'm so happy I could kiss you!" he told Luna.
"SHE…" and angry James pointed at Luna, "We would have come sooner, but SHE made us swear unbreakable vow!"
"I'm sorry!" said Luna, looking at James with her large grey eyes, "there was no other way."
"So, how about that kiss?" murmered Luna, wrapping Harry in a hug.
(And yes, Frank & Alice get healed in the after-credits.)
Every Harry-loyal intelligent time-traveler who gets stuck in 1980 Britain with the cannon JK Rowling time-travel constraints (Don't Change What You Know Will Happen) should do this. It's hard to believe it would be genuinely impossible. Frustration that it doesn't happen in several (otherwise very good) time-travel stories drove me to write this, so take that as you will.
I do not, of course, own Harry Potter. The idea of changing the timeline without changing the timeline was executed before me by DebsTheSlytherinSnapefan in Fixing Past Mistakes, and probably by others too.