Bellatrix Lestrange and Nightwing would be an unexpected duo. Bellatrix is a deranged Death Eater, and Nightwing is a symbol of good in Gotham. But if Voldemort made a deal with a Gotham super - villain to take over both worlds, and Bellatrix was sent to work with Nightwing as part of the deal, they would have to find a way to coexist, at least temporarily. Their interactions would be full of tension and power struggles.
I think Luke Skywalker and Olaf would be quite an unlikely pair. Luke is this serious Jedi Knight, trained in the ways of the Force, while Olaf is a carefree, snowman who loves warm hugs. But perhaps Olaf's innocence and positive attitude could teach Luke to not be so burdened by the weight of the galaxy all the time.
I think Loki and Crowley would be quite the unlikely allies. Loki is known for his trickery and self - serving nature in the Avengers universe, and Crowley from Good Omens is also a demon who loves to cause a bit of chaos. They could team up to play pranks on both the Avengers and the forces of Heaven and Hell from Good Omens. Maybe they'd create illusions to mislead everyone and have a good laugh while doing it.
Another unlikely pair could be Percy and Hulk. Hulk is all about brute force and anger - driven power. Percy is more in tune with his strategic thinking as a demigod. But if they were in a situation where they had to fight a huge, powerful enemy, like a giant from Greek mythology that has been enhanced by some sort of modern technology, Percy could use his water to cool Hulk down when he gets too angry and Hulk could use his strength to physically take on the enemy. Together, they could be a surprisingly effective team.
A really cool storyline would be if there was a magical artifact in Gotham that was connected to the wizarding world, and Nightwing and Harry team up to retrieve it. Nightwing's acrobatic skills and knowledge of the city streets combined with Harry's magic would make for an exciting adventure. They could face off against both magical and non - magical foes along the way. Maybe the artifact is being sought by Voldemort and one of Gotham's super - villains, so they have to work together to stop it from falling into the wrong hands.
A Harry Potter Saiyan Warrior fanfiction could be really interesting. Imagine Harry Potter having Saiyan powers. He could use his Ki to enhance his magic, like shooting a super - powered spell with the energy of a Kamehameha. It would completely change the dynamics of his battles against Voldemort and his Death Eaters. He could also train like a Saiyan, doing intense physical workouts to increase his power level, which would be a new concept in the wizarding world.
Mercy from Overwatch and the Medic from TF2 would be likely allies. They both have a strong focus on healing. Mercy's ability to fly and revive teammates could be well - complemented by the Medic's Ubercharge ability. Another pair could be Tracer and Scout. They are both fast - moving characters. They could use their speed to scout out areas, steal intel or make quick strikes against the enemy.
I think Mercy from Overwatch and Medic from TF2 could be great allies. They both have a strong focus on healing and could share medical knowledge and techniques from their respective universes. Mercy's ability to fly and revive teammates could be combined with Medic's ÜberCharge in interesting ways during battles.
Hermione would likely be one of his allies. She's intelligent and has a strong sense of justice, always having been interested in the rights and wrongs of the wizarding laws. She could provide Harry with valuable research and advice.
Dumbledore could be an unexpected ally. He might see this as an opportunity for Harry to stand up for himself and gain some closure on his difficult childhood. Also, some of the Hogwarts professors, like McGonagall, could be on Harry's side. They have always cared for Harry's well - being and would not tolerate the Dursleys' abuse. And there could be some new magical characters introduced specifically to help Harry in his legal battle, like a wizard lawyer who has a soft spot for underdog cases.
Goku would be an obvious ally for Captain America. Goku is always eager to fight for justice and protect the innocent, which aligns well with Captain America's values.