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.
Dr. Strange and Aziraphale could be an unlikely pair. Dr. Strange is a powerful sorcerer in the Avengers world, used to dealing with threats on a large scale. Aziraphale is an angel from Good Omens, more accustomed to the subtleties of moral and spiritual battles. However, they could find common ground in their knowledge of magic and the supernatural. Aziraphale could teach Dr. Strange about the more angelic aspects of magic, while Dr. Strange could show Aziraphale some of the more practical uses of his powers in combat situations.
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.
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.
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 Klaus and Loki could be likely allies. They both have a sense of mischief and are often underestimated. They might team up to outwit their more 'righteous' counterparts. Klaus has his vampire powers and Loki has his Asgardian magic and trickery.
Inuyasha and Thor could potentially be allies. They both have a sense of honor and are very powerful warriors. Thor with his hammer and Inuyasha with his sword could fight side by side against powerful enemies. Also, Captain America and Inuyasha might get along. Captain America would respect Inuyasha's fighting skills and determination, and Inuyasha would see Captain America as a worthy leader.
I think Sans and Iron Man could be great allies. Sans is intelligent in his own way in Glitchtale, with his knowledge of timelines and shortcuts. Iron Man is a tech genius. They could combine their knowledge to create strategies against common enemies. Iron Man might be intrigued by Sans' magic - like abilities and try to find a scientific explanation for them. Sans, on the other hand, would be impressed by Iron Man's high - tech gadgets.
I think Aziraphale and Lucifer would be fascinating. Aziraphale is an angel who loves earthly pleasures and has a soft spot for humanity. Lucifer, being the fallen angel, would view Aziraphale's views as naive at first. But as they talk, there could be some deep - seated realizations on both sides about the nature of good and evil.
Sam and Aziraphale could also be an interesting pairing. Sam is the more intellectual of the Winchester brothers, always researching and looking for answers. Aziraphale is an angel with a vast knowledge of the occult and religious lore. They could team up to research a mysterious supernatural phenomenon that neither of their respective universes has seen before. Sam could learn about some of the more angelic aspects of the supernatural from Aziraphale, and Aziraphale could be impressed by Sam's modern - day research methods.
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.