Harry Potter and the Slayers Part 0: Story Structure. If you do decide to do this story idea do each season one at a time by making each season a sequel story to the previous so you do not burn yourself out. Also no fourth wall breaking! The world and story should be immersive and fourth wall breaking puts a screeching halt on that! Also find the Novels the franchise was based on and take into consideration the OVA's of the Slayers series and some of the Fan based works on this sight

Harry Potter and the Slayers Part 1: Harry's powers and abilities and Harry's Adventure's. Harry has to be 8 years old when he get's dragged into the Slayers world(Ceipheed has to be the one who brought Harry into the Slayers world after Harry cries out for help which the Flare Dragon responds and answers that prayer) where he found by Luna and Lina Inverse and is taken in by the Inverse Family as a son and for his adventures he has to go through the first 3 seasons of the Slayers Anime or the first 8 volumes of the Slayers Light Novels and some of the various Slayers Manga until he turns eleven.

During this time Harry has to develop all the skills of a mid tier sorcerer at the start of his adventures in the Slayers World, with Black, White, and Shamanistic Magic under his belt and even learns Holy Magic from Philia and The Golden Dragons who lived at The Fire Dragon King's Temple so Harry is not as vulnerable or inexperienced and when he returns to Hogwarts for his Wizarding training and adventures where he gains all his cannon skills and abilities but Harry has to develop his cannon personality but now has a genius level intellect and an incredible drive to learn and is willing to work hard to gain new skills and has incredible self control. Also when Harry turns eleven he has to go through the events of Harry Potter and the Sorcerer's Stone.

When you get to the 4th and 5th Seasons of the Slayers Anime Harry has to develop all of the skills that Lina and Zelgadis Greywords have excluding the Giga Slave and Ragna Blade spells but Harry learns Gourry and Zelgadis's swordsmanship just to give him a better chance without his wand along with learning Amelia's skill set and every thing Filia can do with Holy Magic in cannon as well. Also it would be beneficial for Harry to have that Horcrux in his scar exorcised after Lina finds away to milk all the information of Tom's Riddle's wizardry before he became Voldemort and some of the locations of the other Horcruxes so Harry does not have to martyr himself and to gain a bit of a jump on those who want his head. Because whew boy! Lina and the rest of the Slayers would go ape shit crazy and demolish Magical Britain after developing a big sibling and student/teacher relationships with Harry if he does martyr himself!

After Harry completes his adventures in the Slayers World he goes through the events of the remaining Harry Potter Books after the Sorcerer's Stone. Now in Harry's later school years Harry learns Norse Runes and other Norse magic's like Gandr and Seidr, he also learns some European and Asian magic's like Taoism, Omoyudo, Celtic Magic, Western and East Asian Alchemy, and some really old powers like Ancient Egyptian magic's and other anime series magic like all the powers of the Dark Magician and it's various counterparts across the Yu-Gi-Oh Franchise, The magic's of Card Captor Sakura series, other powers in HP cannon like being an Animagus when he learns his parents were ones and his Animagus transformation makes him take the form of a gold and black Western like European Dragon with traits of a East Asian Dragon(Two long horns on his head with silver hair, with it being a reptilian quadruped with four legs, Two Wings, a long tail that ends with an arrowhead but with fur on the end of it and with the whiskers of a Chinese Dragon with near impenetrable scales that are imbued with the energy of Ying and Yang and every scale having enough mystical power to light up the world for 100 years with all of the abilities of the Golden Dragons and Ancient Dragons)and with all the powers of real world Dragon Lore and Mythology from across the globe and for Harry to be something of a titan of magic and a innovator Harry discovered and devised the magic systems found in the Nasuverse of Fate/Stay Night and worked to merge science and sorcery like in the Magical Girl Lyrical Nanoha series of magic as he invented it and developed it just to give the Wizarding World and Slayers World a light of hope to strive toward with Harry becoming the next Merlin for all intents and purposes for the Wizarding World and Slayers World and did not give up his humanity in order to grow to be something awesome.

Harry Potter and The Slayers Part 2: Magical Migration. The 2nd part of the challenge is that eventually the Wizarding Communities of Earth cannot stay in hiding forever along with many of those Magical Creatures so it would be beneficial for the Wizarding Communities and it various Magical Creatures to migrate to the Slayers World Lina lives in for everybody's benefit. It is a somber truth but one that needs to be addressed but it would be for the betterment of everyone and many of those Fantastic Beasts would have a place where they can roam free with the sentient magical beings being treated as equals to wizards and magically aware humans but still have the protection that they had in the HP universe and maybe give a technological boost to the Slayers world as we have seen in the Fantastic Beasts and Where to Find Them movies along with the technology in Harry Potter's time period making an industrial boom but not a big enough one that it causes global warming or climate change and make a world were science and magic coexist. Also the Wizarding communities of Earth can assimilate exist in peace with the Sorcerers of the Slayers world without persecution and they can have their culture and lifestyle without needing to live in hiding.

Harry Potter and The Slayers Part 3: Harry's Equipment. For Harry's Equipment it is almost all cannon. His cannon wand and eventually The Elder Wand, his Cloak of Invisibility, The Resurrection Stone and his second broom. It wouldn't be Harry Potter without those key items and equipment as Harry would not be himself otherwise. Also Harry makes his own sword that has the same powers as the Sword of Light in Slayers but being able to channel Harry Potter and Slayers magic and spells through the Blade and like the Sword of Gryffindor is able to only take in that which makes it stronger. Also speaking of wand's Harry finds a way to salvage the wood and cores of his original wand, the wand he steals from Draco Malfoy, and The Elder Wand to make his Staff of the Magi that can aid Harry in casting Slayer universe and Harry Potter spells so he can have two of his own signature mystic weapons. The Staff of the Magi has all the cannon powers of Harry's original wand, the Elder Wand, and the Wand he stole from Draco Malfoy.

Harry Potter and The Slayers part 4: Harry's enemies. Harry's enemies should not be lightweights. The first few enemies that Harry encounters are the one's that the Slayers meet and face in the series. Now when Voldemort learns of Harry's new powers and of the world he learned those new skills he finds away to the world of the Slayers that Lina lives in and learns the skills of the best wizards and sorcerer's there and delves into making himself into a high-ranking Mazoku with all of Voldemort's cannon powers, The Magic's of the Slayers world and the powers of a high ranking Mazoku along with the powers of a Vampire, Bogart, and Dementor, along with learning other Dark Magic's such as Voldemort learning Necromancy, Norse Magic, Celtic Magic, Dark Greco/Roman Magic, Alchemy, Ancient Egyptian magic and gain the powers of the Dark Creatures that he pit's against Harry in cannon to bump himself up the food chain and become the monster the Wizarding World sees him as and a legitimate threat. Voldemort should have the venom of the Basilisk as his blood and in his teeth which can kill most people in a single bite, the killing gaze of the eyes of the Basilisk which he can turn on and off at will, drain the blood, souls, and all emotion and happiness of his victims like a Dementor and Vampire to power himself up and recover from injuries, and to restore his magic and energy from the life force of his victims. This guy has to turn into an abomination that people have to have nightmares about.

Voldemort also now has the brute strength and magic resistance of a HP Troll and HP Giant and Voldemort can also take the form of what his opponent fears most like a Boggart and draws strength from fear with the more people are afraid of him the stronger he becomes, and like a snake he can shed off his previous body to restore himself at his best but only with intense concentration so it is useless in a fight. Voldemort now also has a healing factor and can also regenerate his body without outside help such as limbs, bones, and vital organs but it takes time and like Bram Stokers Count Dracula, Voldemort can turn into mist, a flock of bats, take the form of a wolf, become younger and stronger the more blood he drinks and can resurrect himself on certain occasions has found out how to make the his version of the Philosopher's Stone(which his is a fusion of the Harry Potter and Slayers Philosophers Stone's)and has drunk the elixir of life and if you include Voldemort's Horcruxes he will not be able to die unless those are destroyed along with burning his coffins and make him near impossible to put down.

Voldemort when he get's the Elder Wand and goes looking for the other Deathly Hallows should also learn to use dark matter, and learn dark voodoo and grows stronger the more death he deals and the more fear their is in the area he is in. Voldemort to be a better foil to Harry Potter can take the form of a Feathered Winged Serpent to mock Harry's new Dragon Form and also become a humanoid Werewolf like in the Harry Potter movies but be in control of himself in his werewolf form and take that form at will at any point of the lunar cycle but also have the traditional weaknesses of Werewolves like Silver bullets which can hurt him and the herb Wolfs Bane which can as well an seeing as Voldemort has become a pseudo vampire he has problems moving in direct sunlight, which drains him but does not kill him just to not make him to overpowered. Voldemort also has to make deals with literal Demons for more power and knowledge of magic ala Faust to make him into a true Dark Lord along with his most loyal lieutenants among the Death Eaters also evolving and the same with their allies evolving just to make them a credible threat against the threats in the Slayers World and the Slayers themselves.

Harry Potter and The Slayers part 5: Parings. Harry/Hermione, Harry/Fleur or Harry/Ginny. Zelgadis/Amelia and Lina/Gourry. If Harry does get with Hermione Ron has to get Fleur Delacour. If Harry gets with Fleur, Ron gets with Hermione. If Harry gets with Ginny Ron gets with Hermione. Luna/Neville Longbottom. No harems or gender bending!

Harry Potter and the Slayers part 6: Character portrayals. As every one in the story is their cannon self they have to be represented in a believable way. That means no character bashing on the Harry Potter or Slayers sides of the crossover! I repeat! No character bashing on the Harry Potter or Slayers sides of the crossover! Everyone is their cannon self or as possible with this crossover! Deconstruction maybe but only to reconstruct them later.