Now if you do this challenge I expect their to be a happy ending! These guys have been through to much to be given the short end of the stick! Now for the Title, you can do whatever you want the same with parings so long as it fits their cannon characters go for it! Also if you do want to do this challenge I recommend you do some more research into the works of Type Moon. I cannot stress this enough. Please read the Fate/Stay Night Visual Novel and their cannon supplementary materials and get the Fate/Grand Order game on Mobile as well as look at the uncensored YuYu Hakusho dubbed anime, read the manga and watch the movies and their cannon supplementary material!
Fate Stay Night YuYu Hakusho crossover part 1: The Story's Beginning.
Now the premise of the story is that Shirou Emiya has hit a wall in his Magus training and during the arch were Yusuke is resurrected and begins his job as a Spirit Detective in tracking down the Three Artifacts of Darkness Shirou Emiya seeks out Genkai to help him at least push him in the right direction he transfers to a High School their for at least a year before going through The Holy Grail War under the pretense of trying to see the world and move on from his father's death. When in Tokyo Shirou has to become Yusuke's partner after seeing him getting hurt against Goki and being the good Samaritan and wannabe Hero of Justice that he is, Shirou buys Yusuke Urameshi the time he needs to put him down. Now during his time in Tokyo Shirou has to go through the first three seasons of the YuYu Hakusho anime and the Sensui Arc to truly stand a chance later against the opponents that he faces in the Holy Grail War and develops a strong familial bond with the Spirit Detectives and their allies so he can call them for backup if he needs it in the Holy Grail War. Also those experiences with the Spirit Detectives might just give Shirou a chance to become more human and shake off some of his more distorted qualities as he develops along side Yusuke and the crew.
Fate Stay/Night YuYu Hakusho crossover Part 2: The World of the Story.
Now before you say that the YuYu Hakusho universe cannot happen or crossover with Type Moon due to most of the gods are either dead or unable to walk amongst mortals in the Type Moon Universe, the realm of Buddhism is still very active as we see with Saver from Fate Extra and the Shinto Gods like Amaterasu are still kicking and physical beings they just don't interfere. so it can happen as the Spirit World of YuYu Hakusho is Buddhist. Speaking of which the Buddhist, Shinto and Chinese Gods are bumped up to their Type Moon counter parts levels of power and the same applies with the S-Class Yokai to match them. Also in the Type Moon Universe their are psychic's that have magic circuits in their eyes and in the YuYu Hakusho universe their are psychic's that can master pure spiritual energy(see Genkai). Also the Heavens Feel can be the Ultimate Mastery of Spiritual Energy in both Magic and Spiritual Pressure. Now in this story their is an Eastern Mages Association but those mages get their power directly from the Shinto, Hindu, Chinese, and Buddhist Gods without following the system of Modern Magecraft like Western Modern Magi and it is also why Western Magi hate Japan and The Far East and it also reminds them that mankind is not the dominant force on the planet. The Eastern Mages Association must be far more moral than organizations like The Clock Tower and the Western Mages Associations and work closely with the Spirit World, the Far Eastern Gods, and the Spirit Detectives, However, Alaya and Gaia are vastly diminished in this world and even more so in the Far East hence why their is still a lot of Supernatural Incidents in The Far East and why are heroes have their powers. Also the middle eastern and Gods of the West can no longer take physical form and like Ishtar have to find specific hosts to take over or exist in the far side of the world. The Demon World that the Yokai inhabit can be a separate plain of existence that is not the Reverse Side of The World but a realm of Yokai that they created for themselves and no one else.
Fate Stay Night YuYu Hakusho crossover part 3: Shirou's strength and abilities. In the YuYu Hakusho part of the crossover Shirou when training under Genkai has to activate his natural Magic Circuits, learn how to use Norse Runes, create Spirit Blades like Kuwabara and eventually be able to gain his cannon powers such as using his Reality Marble to trace blades and other weapons, be able to channel pure Spiritual Energy and Prana and Od through a bow to create energy arrows, and channel his spiritual energy through his traced swords and weapons to strengthen them and be able self destruct them by over loading them as well as be able to use Sacred Energy but not on the same level as Sensui or Yusuke later. Shirou in terms of YuYu Hakusho standings has to be at a S rank like the rest of the Spirit Detectives when using his spirit energy at the end of his time in Tokyo with the Spirit Detectives but be a third rate Magus in the terms of Clock Tower's standing with average Magic Circuits and have his cannon total of twenty seven Magic Circuits. Now when you get to the Holy Grail War Shirou begins to improve his Magecraft Skills and unlocks his Reality Marble more and more until he is able to unleash it in full force on his own and also after beating Archer EMIYA he has all the skills of his Alternate Counter Guardian self allowing Shirou to reach his full potential and for him and The Spirit Detectives to be recorded on the Throne of Heroes when their time comes without Alaya or Gaia's influence but by skill and hard work alone.
Fate Stay Night YuYu Hakusho Crossover Part 4: The Fate Route. When Shirou goes back to Fuyuki City for the Holy Grail War Shirou has to go through the Fate Route with elements from Unlimited Blade Works and Heavens Feel. The Fate Elements are Shirou falling in love with Saber, Shirou facing off against Kotomine, and having Saber destroy the Holy Grail with Excalibur with Illya being alive at the end with Sakura and Illya being saved from their cruel fates. The elements from Unlimited Blade Works are Shirou manifesting his Reality Marble and facing off against Archer when he backstabs Rin and joins Caster to kill Shirou and having Gilgamesh destroy Berserker but Illya survives the encounter due to Shirou successfully pulling of a Big Damn Heroes Moment. Now when Sakura finds out that Shirou has fallen in love with Saber, That Shinji was used to summon The Holy Grail, and that Medusa was killed, she finally snaps and becomes Dark Sakura and Shirou has to use everything in her power to save her life and arrest Zouken. This arc has to be Shirou's greatest test of his powers, skills, and his ideals. But he does not have to do it alone as he has allies among the Spirit Detectives human allies.
Fate Stay/Night YuYu Hakusho Crossover part 5: The Spirit Detectives Grand Order. After the Holy Grail War and the Three Kings Arc, Shirou, Yusuke Urameshi and the Spirit Detectives are forced to do one last job as Spirit Detectives. The Spirit Detectives and Shirou have to go down the events of the Storyline of Fate Grand Order that revolve around the revival of King Solomon. And at the end, they prove that the world still does have heroes, and that they are the harbingers of a new age of heroes, combining the old and new.
Fate/Stay Night and YuYu Hakusho crossover Part 6: The Spirit Detectives new powers. Each of the cast have to gain all their cannon powers and abilities as well as new powers from the Type Moon side of the crossover.
Kuwabara has to learn Oriental and Japanese forms of Magecraft like Senjutsu, Taoism, Buddhist Sutras, Tracing, Reinforcement, and Gradation Air as it would fit his concept of him having high spiritual awareness and the human members learning Magecraft would give them some more variety from cannon and considering Genkai can use her spiritual energy in a similar manner and you can even give Kuwabara some original abilities to go with his character and if you add Senjutsu it would fit as well as it involves using your own inner energy along with the energy of the world around you.
Kurama should learn how to use Yojutsu, Plant based Magecraft, Taoism, Illusion based Magecraft and Eastern Magecraft just like Tamamo No Mae as those powers would fit him quite well. Kurama is a Kitsune who are know to be both malignant and benevolent, wise, tricksters, and masters of illusions, and Illusion magic as Kitsune are masters of illusion in legends so it would fit him. Add that to his concept as a master strategist and add his back round as a master thief and bandit, with him knowing Yojutsu and Taoism you can see the enemy literally walking into his trap and following Kurama's plans every step of the way like a boss. Also he should learn to use Senjutsu like Kuwabara.
For Hiei he should learn how to use Ice based Yokai Techniques as a weapon along with his Fire Yokai based abilities, Taoism, and obtaining the powers of a Type Moon Dragon and Prana based techniques that Mordred and Arthuria are able to do because of him having a Dragon Motiff. The reason Hiei has to learn how to use ice based attacks goes with his back story, we know that Hiei was born to a clan of Yukki Onna's called the Kuromihe, and since he was born a Fire Yokai he was cast out, and grew up with a grudge against his birthplace and yet a longing for a home. Hiei learning how to use ice must happen by making it an arch about Hiei coming terms about the past and forgiving those who ruined his life while training so he can come full circle on his revenge trip.
For Yusuke Urameshi he should learn to master Sacred energy and all the abilities Sensui had, the abilities that Yusuke has in cannon, and obtain all the same powers of the Spirit Detectives(aside from Shirou and the new Magecraft powers of Team Urameshi)have and gain the powers of a Type Moon Phoenix as Yusuke has gone through a cyclical series of deaths and resurrections. The reason it should happen is this. At the beginning of YuYu Hakusho, Yusuke Urameshi is a punk with no friends and constantly gets in trouble and only really looks out for himself at the beginning in the series. Yusuke learning all the skills and obtaining all the powers mentioned would show how far he has come from that punk he used to be by him learning discipline from training, how to use his mind by studying and meditating, how to value life from his near death experiences, how power should be used because of all his fights and value the world around you by traveling and solving cases and show the bonds he made throughout his journey and give him the power he has to overcome any obstacle in his way just like cannon.