This idea has been bouncing around for a while, and new stuff keeps coming up randomly from all across the HP timeline. Here's the beginning. Review for how you like it, because this could easily become a proper story later.
I do not own Danny Phantom or Harry Potter. That doesn't stop me from being inspired by them, though!
Danny was walking home from the Nasty Burger with his friends when a wisp of blue mist escaped his mouth; not at all an unusual occurrence since that was his ghost sense and there were often attacks when he was just walking around town. He automatically dropped his backpack, which had been hanging lazily from only one shoulder, and ducked into an alley. Sam and Tucker, his two best friends and most trusted companions in the entire world, moved to block the alley entrance from view. Tucker also picked up the dropped backpack and, with practiced ease, pulled a metal thermos and two ecto-guns from it. A flash of light came from behind them in the alley and suddenly Danny Phantom leapt out, taking the proffered thermos in hand. But the ghost that floated trough the wall of the building on his left was one he had never seen before; in fact it was one of the strangest ghosts he'd ever seen, which was really saying something.
The ghost in question was like a different type of ghost entirely. For one, he was a pale gray color and transparent. For two, he looked entirely human—other than the fact that you could see through him, that is—because he didn't have any weird features like animal parts or fangs or flaming hair or any of the other odd qualities a ghost picked up. Even if he did appear to be wearing a dress, or robe of some kind... He didn't even glow, and that was something that all the ghosts he knew did, including himself. He was more like a person that wasn't all the way there. The other weird thing was that the strange new ghost wasn't attacking him. At all.
Danny glanced at his friends, who shrugged. "Um," Danny chanced, deciding to attempt talking to the ghost, "who are you?"
He floated forward and spoke, though he did not respond to the question. "My friends need help. Please, you are strong. I have seen you fight. Will you help them?"
"Sure, I guess, but what-"
The ghost, apparently needing no further permission than that, put a hand into a pocket of his robes and pulled out a long, thin stick about the length of his forearm. Tucker gasped. Sam didn't get much further than "What?" before a beam of white light left the tip of the stick and shot to Danny's chest, faster than he could react. The half ghost suddenly found he couldn't move, not even to speak, and his eyes were riveted to the stick (a stick of all things) that he'd just been shot with. "With this power granted me after life, I bind you, hero, to aid Harry Potter in the war against Lord Voldemort."
Sam, utterly confused and seeing no way to help her best friend, looked over instead to Tucker to see if he had any ideas. Instead she found such a look of utter shock on his face that she was even more confused. Why would Tucker react like that? But before she could question him, the strange ghost disappeared with a sigh and Danny was suddenly unbound.
"What in the world was that?" Danny said as he turned toward his friends. Sam seemed just as confused as he, but Tucker's face... "Hey, Tucker are you okay?"
"That . . . that was a . . . a ghost from . . . ."
"From where, Tucker? We see ghosts almost every day. Why are you acting weird?"
"That was a wizard ghost!"
Sam and Danny glanced at each other. "Did you say 'wizard'?"
"TIME OUT"
The trio suddenly found themselves with a medallion around each of their necks and the Master of Time himself floating before them. Time had stopped for everything around. ". . . Hi, Clockwork?"
Clockwork sighed, as if he were very tired. "Daniel, what just happened is extremely serious."
"You mean-"
"That strange ghost, yes. As your friend Tucker just stated, that was not an ordinary ghost, at least not the kind you're used to. That was the ghost of a wizard."
"When you say wizard . . . ."
"A person who performs magic." Danny opened his mouth. "The weapon he shot you with is called a wand, and it is the focus wizards use to control their powers." Clockwork looked to Tucker. "I am aware of the laws, but this is the time your friends should be told."
"Well . . . we call it the wizarding world. It's mostly separate, because almost no Muggles—uh, non-wizards—know we exist."
"Then how do you know?" Sam asked.
"I kind of am one," Tucker said sheepishly as he pulled a thin stick from his pocket. A wand. Danny and Sam stared. Tucker had a wand? He was a wizard? "I go to school with you guys because I'm home-schooled in magic."
Danny and Sam could apparently think of nothing to say. Clockwork, who knew this, stepped in. "Daniel, you must listen now. An extremely unlikely sequence of events just fell into place and you have been bound with a power of the Ancients. What just happened cannot be undone, not even by me."
"But what exactly . . . ."
"When a wizard dies, he or she has the choice to become a ghost of the type you just saw. Typically these wizards and witches die violently and choose to become ghosts to complete unfinished business of some kind. The wizards who choose this are rare. Once ghosts, they are given a second choice. Most choose to haunt a place or person. A few others choose a purpose. The Ancients give them a special power so they may carry out their purpose, and once they do, they pass beyond. It is one of these that you have encountered."
"So this ghost, what, bound me to, to help someone?" Danny said, trying to remember just what the ghost said. "With a war!?"
Clockwork waved a hand and a floating screen appeared, showing a frozen Danny and the wizard ghost holding his wand. With this power granted me after life, I bind you, hero, to aid Harry Potter in the war against Lord Voldemort, he said again, and then vanished, followed shortly by the screen itself.
"Who's Harry Potter?"
"That's the Boy Who Lived!" Tucker said. Sam, Danny, and Clockwork looked at him. "Every wizard around the world knows about him. Voldemort tried to kill him, but he failed. No one knows why. The Killing Curse is supposed to be impossible to defend against." Tucker turned to Clockwork. "But didn't Voldemort die? He was never seen again! How can there still be a war against him?"
"Voldemort did not die," Clockwork said, and Tucker's face paled. "He was very near death for a long time, but he did not die. And he has recently returned." He turned back to Danny. "Harry Potter, and the war against Voldemort both, are in Europe. The boy and the school he goes to are both in Britain."
"If I have to help them . . . ."
"You must go to Britain, yes."
"WHY!?"
"Because, Daniel, you have been bound by a power of the Ancients. The compulsion laid upon you will force you to aid Harry Potter in the war, and to do that, you must be where he is. Whether you go willingly or with the utmost reluctance, you will end up in Europe. Now, as your guardian, I am giving you an opportunity. You may go, unwilling or not, to Europe as you are."
"What, now?"
"Or, you may go there as a student and begin education as a wizard. If you choose this, I will turn back your timeline so your are the appropriate age to begin attendance as a first year."
"Turn back my timeline?"
"I will reverse your personal timeline until you are age 11, and I will send you back in time the same amount so you may attend Hogwarts."
"Are you allowed to do that?" Danny gave Clockwork a skeptical look.
"A very unique set of conditions have been fulfilled."
When Clockwork elaborated no further, Danny asked, "If you make me 11 again, won't that take away my ghost powers? Since they won't have happened yet?"
"Yes, but only temporarily. You will regain your powers on the same day you gained them originally, and they will develop the same as before. This is because I am not turning back all of time itself; only time for you. For a short while, two of you will exist, and you will physically develop and grow simultaneously, until the very day you disappear from here, set upon your second timeline."
"That . . . is kinda complicated. Wait. All these options; they mean I have to go to Europe. That I have to leave Amity Park!" Clockwork nodded. "But what about the ghosts?! Who will protect the town from ghost attacks?"
"You may be surprised, Daniel, at the future of Amity Park during your absence, but it will be protected."
"By who?"
"Your friends and family, of course, as well as a few surprises. But you will find out. In time." Clockwork smiled.
"Why would I want to be 11 and go to this strange school? Why not go now and deal with it?"
"Because you do not just 'deal with' a war, Daniel. And the advantage to going back in time for you would mean the chance to learn about the wizarding world and how it works, saving you much pain and effort than if you just went immediately."
"What would happen?"
"I cannot tell you Daniel. You must choose, and you will find out."
"Can I even go to a wizard school? I'm not a wizard."
"Actually, Daniel, you are. All three of you are wizards, though you and Sam never got schooling due to your parents." The trio just looked at him. "Your parents," Clockwork said, still looking at Daniel, "thought the letters were a joke teasing them about their study of ghosts, and they ignored them." He turned his gaze to Sam. "Your parents refused to indulge such nonsense and your letters were also ignored."
Everyone looked at the half-ghost, who appeared to be pondering his feet. Finally, he said, "I suppose learning magic wouldn't be that bad. But everyone here—my parents, everyone at school—won't they wonder where I've gone? If I just disappear, they'll assume I got kidnapped, or, or killed in a ghost attack, or something, won't they? And what about the disappearance of Phantom? People will wonder what happened to him, too."
"Daniel Fenton can accept a scholarship to a foreign boarding school. Danny Phantom, however, will seem to have disappeared, as suddenly as he came."
"Why are you offering to do all this just for me?"
"Because I am your guardian, Daniel. Ever since I gave you a second chance," Clockwork glanced over at the dented thermos, still in the same condition as when it had captured the infamous and feared Dan Phantom of a timeline that ceased to exist, "I have been charged with personal responsibility by the Observants for your timeline."
For several moments, Danny was silent.
"I'd accept," Sam said suddenly in the silence of stopped time. Danny and Tucker both looked at her. "I mean, you have to go no matter what, right? Because it's some sort of compulsion?" She glanced at Clockwork, who nodded to confirm her choice of words. "And if you accepted, you'd get to learn magic. Who wouldn't want to learn magic? Right, Tucker?"
"You do learn some really cool stuff," Tucker confirmed. "I could never give up technology, though. Wizards don't really use technology much at all. My parents don't even own a computer!" Tucker said this as if it were a major tragedy.
Everyone waited for Danny's response, which came eventually, and with a reluctant sigh. "Okay. How long will I have?"
"Your scholarship offer will arrive in the mail tomorrow. You will leave on August 30th. This will give you the 31st to buy your school supplies. School begins on September 1st."
"How am I supposed to get there?" Danny said in a voice that made him seem merely resigned to the whole thing.
"You will go to the airport. There, I will retrieve you and we shall go to Britain together."
"Will I still be able to talk to Sam and Tucker? If you're sending me back in time?"
"I will ensure your communication can continue."
"Thanks Clockwork."
"Do you have any more questions?" Clockwork asked to be polite, even though he already knew the answer.
"How long will I be gone? From here, I mean? You said it will seem like I go to boarding school."
"That I cannot tell you, though attending Hogwarts means seven years of schooling."
Suddenly hopeful, Danny grinned. "Hey, if you turn back my timeline, won't that mean that the, the compulsion thing won't have happened?"
"No. Its power comes from the Ancients, and it's effects will transcend time. However, if will affect you very little until the day it was applied, this day, comes again."
Danny frowned again, as if he'd just lost a treat. "How will it affect me?"
"It will pull you toward making certain decisions. Unless they are unusual decisions, something you would not have normally done, you may not notice it acting on you at all."
Danny had run out of questions, so Clockwork turned to Sam, who had some. "How will we be able to talk to Danny? If he's in a different country, at a wizard school, and in the past?"
"Being in the past will not be a problem. And Tucker will be able to help you, for wizards communicate most often by owl, and his family owns two."
"You have pet owls, Tuck? I never knew that . . . ."
"My parents usually take care of them, and are the only ones who use them. I never have, except for maybe twice. So they stay in their room, when they aren't out flying around to catch food and deliver letters and stuff."
"Clockwork, didn't you say I was a wizard too? Is there some way I can learn?"
"Because you were never accepted formally at a school, you cannot go to one now, but you may ask Tucker's family for private instruction. They can even help you get a wand."
"You keep talking like we can't go with Danny. Can't we go?"
"As I am Daniel's guardian, and the compulsion is laid only upon him, I can bend the rules for him, and him alone." Clockwork turned away from Sam and Tucker and his gaze returned to Danny once more. "I will leave you with your friends now. Until the 30th of August. TIME IN."