Kagome's spiritual arrows could be enhanced by Grell's death scythe in a strange way. Maybe their combined powers could create a new weapon or a powerful blast that could defeat a very strong foe. Also, Shippo's illusions could be used to deceive the enemies, and Bardroy's explosives could be set off at the right moment to create chaos among the enemy ranks, which would give Inuyasha and the Black Butler characters an advantage in battle.
Ciel's ability to command Sebastian and his other servants could be applied in the world of Inuyasha. He could direct Inuyasha and his friends in a strategic battle plan. Meanwhile, Sango's weapon, Hiraikotsu, could be used in tandem with Mey - Rin's marksmanship skills. Sango could throw Hiraikotsu and Mey - Rin could use her guns to guide it or create distractions around it, making it a more deadly weapon.
Inuyasha's physical strength and combat skills with his sword could be combined with Zatch's magical attacks. For example, Inuyasha could use his speed to get close to an enemy while Zatch casts a spell to immobilize or weaken the foe. Then Inuyasha could deliver a powerful strike with Tessaiga. Another way is that Zatch could use his magic to enhance Inuyasha's attacks, like making Tessaiga's wind scar even more powerful or giving it additional elemental properties.
Inuyasha's demon blood could interact with the devilish powers in Highschool DxD. His berserker mode when his demon blood takes over could be enhanced or controlled by some of the devil's magic. On the other hand, the characters in Highschool DxD could learn from Inuyasha's group about fighting without relying too much on modern technology. For example, the simple yet effective fighting styles of Sango and Miroku could inspire some of the characters in Highschool DxD to improve their hand - to - hand combat skills.
Inuyasha's sword, Tessaiga, could be used in interesting ways in the Fruits Basket world. For example, if a Sohma is in trouble during their transformation, Inuyasha could use Tessaiga to protect them. On the other hand, the Sohma's zodiac animal abilities could assist Inuyasha in his battles. Like Kyo's cat - like strength and agility could help Inuyasha in a fight against a particularly fast demon.
Yusuke's Rei - gun from Yu Yu Hakusho could be combined with Inuyasha's Tessaiga. For example, Yusuke could use his Rei - gun to enhance the power of Inuyasha's Tessaiga slashes. This would create a more powerful attack against demons. The combination of their raw power would be a force to be reckoned with.
Well, Naruto's Rasengan could potentially be infused with black lightning. Imagine a Rasengan crackling with black lightning energy, making it even more destructive. It would be a powerful combination of Naruto's chakra control and the unique energy of black lightning.
Sebastian's superhuman strength, speed, and demonic powers would be a huge asset in Gotham. He could easily take on many of the thugs and villains. Batman, on the other hand, with his detective skills and gadgets, would be very intrigued by Sebastian. Alfred's combat and medical skills might be similar to some of the skills in the Black Butler world. Ciel's intelligence and strategic thinking could also be useful in Gotham's complex criminal underworld.
Naruto's ability to use chakra could be combined with Sebastian's demonic powers in interesting ways. For example, Naruto could use his chakra to enhance Sebastian's physical attacks. Or Sebastian could use his knowledge of the supernatural to help Naruto better understand and control his bijuu chakra.
Inuyasha's demon blood could interact with Naruto's Nine - Tails chakra. Since both are sources of great power, they might be able to create a unique power - up. Inuyasha could use the Nine - Tails' chakra to boost his speed and strength even further, while Naruto could learn from Inuyasha how to better control his inner 'demon' side. Their combined power could be used to break through powerful barriers or defeat extremely strong foes.
The Akumas in D Gray - Man are created by the Millennium Earl. If Akumas were to enter the Black Butler world, they would be a major threat. Their ability to transform and their inhuman strength would be a challenge for the Black Butler characters. On the other hand, Sebastian's demonic powers and his ability to make contracts could potentially be used to manipulate or destroy the Akumas. The D Gray - Man exorcists are trained to fight Akumas specifically, so their skills would be very useful in this new setting. But the Black Butler characters' knowledge of the local environment and their own unique skills, like Mey - Rin's sharpshooting and Bardroy's combat experience, could also contribute to the fight against the Akumas in ways the D Gray - Man exorcists might not expect.
If Naruto was in the Splatoon world, his Shadow Clone Jutsu could be used in interesting ways. He could send clones to cover more ground during an ink battle, confusing the enemy. And for the Inkling's part, their ability to swim in ink could help Naruto move around in ink - filled areas more easily. They could also use their ink guns to provide cover for Naruto while he attacks up close.