I don't own Harry Potter. This one-shot contains content from 'Harry Potter and the Deathly Hallows,' which does belong to J.K. Rowling.
I may not have the time to expand any of these vignettes into full stories, but I invite anyone else to give them a shot. So long as you credit me for the idea, any and all of these one-shots are officially up for adoption.
The Tragedy at King's Cross
"Tell me one last thing," said Harry. "Is this real? Or has this been happening in my head?"
Dumbledore beamed at him, and his voice sounded loud and strong in Harry's ears even though the bright mist was descending again, obscuring his figure.
"Of course it is happening in your head, Harry, but why on earth should that mean it's not real?"
In that instant the bright mist flickered, as Harry stood stock still.
The mist receded on a frowning Dumbledore. "What's that, my boy?"
"If this is both real and in my head, then… how are you here? I'm here because this is my mind. That thing" (Harry pointed at the miniature Voldemort in disgust) "is the horcrux in my scar. But why are you here? How are you here?"
Dumbledore's features flashed with a kind of severity, then resumed their customary twinkle. "Harry, you forget that not even death can overcome the magics of love. You bore me in your heart, just as you bore James, Lily, Remus, and Sirius. And just as you summoned their spirits in your need for comfort before your sacrifice, so too did you summon me."
Harry nodded reflexively, and the mist descended again. But then his eyes narrowed in thought, "But that was the Resurrection Stone! It took a Deathly Hallow to summon my parents. How could I call you without it in hand?"
Again, the mist receded, and Dumbledore took on a disappointed demeanor. "Harry, my boy, this is the Old Magic of sacrificial love. Can you not accept its mystery without indulging your need for explanation?"
Harry bowed his head contritely. But in a flash his mind broke free, running the trail of logic with winged feet.
Dumbledore said this was all in his head, but in the same breath called it real. Dumbledore had died. Yet somehow Dumbledore was really present inside his head. Dumbledore implied it had to do with the Stone, but Harry had used that to summon his parents, and they had disappeared when he dropped it in the forest. Dumbledore didn't want to give him specifics. And frankly, the realization that Dumbledore was inside his head was… actually quite a bit creepy.
He glanced up at the beatific face of his old Headmaster. "Sir, before I return, I'd still like to know."
Dumbledore sighed and removed his half-moon glasses. Rubbing them with a handkerchief he somehow conjured, he began.
"Harry, what you see around you is commonly called a mindscape. When you studied occlumency with Professor Snape, he told you to clear your mind – that is, tamp down on your emotions and stray thoughts in order to access this very place. Now, this place does not merely represent your mind. As you can see by the presence of the horcrux from your scar, it also reflects your soul."
Dumbledore smiled, though Harry found himself increasingly uncomfortable with the look in the Headmaster's eyes. "Now, this is where it gets interesting. As you know, I was struck with a rather fatal curse when I took the ring horcrux from the Gaunt house. But I had a year before it would take my life, a year to prepare myself, and on top of that I possessed both the Wand and the Stone. It took me some time, but I found a way."
Out of nowhere a bludgeoning hex hit Harry in the diaphragm. He collapsed breathlessly, as Dumbledore stood triumphantly before him. "Yes, Harry, you were on the right track. If this is your soul-scape, then there's no way I should be here unless my soul were here as well."
Shackles flew and attached themselves to Harry's wrists and ankles. "Tom was a fool. He attached his horcrux to the ring, not the Stone. But I, using the Elder Wand, anchored my soul to the Stone itself, which could then summon my entire soul after my death on the Astronomy Tower. Then I merely added a trigger to the flesh memory on the snitch, and it was the easiest thing in the world for me to enter and take control of your soul-scape."
He looked down at his sputtering and helpless host. "Of course, it would have been far more difficult to subdue you had your mindscape not been already prepared for me. You can thank Severus for that – really, how did you not realize that the entire purpose of the occlumency lessons was to give Severus a chance to wreck havoc upon this place?"
He leaned down to look Harry in the eyes. "Of course, my boy, you still have your destiny to fulfill. So you must return to defeat Tom, and then we'll finish this tete-a-tete?"
He rose to his full height, wand in hand. "Obliviate!"
The bright mist was descending again…
Author's Note(s): While the line "why on earth should that mean it's not real" was intended as a oblique reference to the after-life, it still struck me as a rather odd expression. Then I wondered what it would mean if it were true, and Dumbledore's spirit was somehow possessing Harry.
I was also struck by the oddity of the Resurrection Stone. Assuming everything that Harry learned in canon were true, how would a real mother react if her son was in that situation? Your son has been made into a martyr and is even now walking to his death. "I'm proud of you" doesn't seem to cut it.
So: Dumbledore made a horcrux out of the Resurrection Stone, possessed Harry (presenting the memories of James, Lily, Sirius, and Remus to draw him in, just as the locket horcrux presented Harry/Hermione to draw in Ron), faded to the background so he could defeat Voldemort, then gradually took over (like the diary that drained Ginny). And nineteen years later, a Harry with twinkling green eyes would send his son Albus Severus to Hogwarts.
Actually, I had quite a hard time deciding how to finish this one-shot. I was initially inclined to expand it, to make it a Battle Royal for Harry's soul to parallel the Last Battle going on in the real world. But then I realized that canon!Harry wouldn't stand a chance against Dumbledore. My instinct was for a happy ending, but then I realized that Harry would need a LOT more prep time before he would have a chance of winning, which would require a more complete AU. So, at least for the one-shot, a tragic ending suffices.