I think Batman and Black Widow would make a really interesting pair. Black Widow has her spy skills and combat training from the Red Room, and Batman has his own unique combat style and detective abilities. They could work together on a mission that involves international espionage and Gotham's underworld. Maybe they have to stop a group that is trading dangerous weapons between the two universes.
Definitely Chat Noir and Iron Man. Iron Man's high - tech gadgets and Chat Noir's acrobatic skills and cat - like reflexes would make for a great combination. They are both a bit of a flirt too, so there would be some really fun banter between them while they're fighting side by side.
Hellboy and Wolverine would be a really interesting pair. Wolverine's feral nature and regenerative abilities combined with Hellboy's brute strength and demonic heritage would make for some great action scenes. They could have some really cool banter too, with Wolverine's gruff attitude and Hellboy's dry sense of humor.
One interesting pairing could be Godzilla and Hulk. Both are powerful, almost indestructible forces of nature. They could have an epic battle at first, as they both have a tendency to lash out when provoked. But then they could end up teaming up against a greater threat, like a powerful alien invasion. Another great pairing would be Godzilla and Iron Man. Iron Man could use his technology to study Godzilla, and perhaps even try to create some sort of armor inspired by Godzilla's abilities. And Godzilla could provide a physical presence that Iron Man could rely on in a fight. Also, Godzilla and Thor could be a fascinating pair. Thor's god - like powers and connection to nature could be paralleled with Godzilla's own primordial power. They might have a mutual respect for each other's strength.
I think Phineas and Iron Man would be an interesting pair. Phineas is full of wild ideas and Iron Man is a genius inventor. They could bounce ideas off each other and come up with some really out - of - this - world inventions together.
Thor and a Dwarf Lord would be really interesting. Thor with his god - like powers and his hammer Mjolnir could team up with a Dwarf Lord, who is known for his skill in forging and combat. They could exchange stories about their great battles and also work together to create some powerful weapon, perhaps using Asgardian and Dwarven forging techniques.
I think Annabeth and Black Widow would be an interesting pair. Both are strong, intelligent women. Annabeth's strategic mind from her demigod training could work well with Black Widow's spy skills. They could be on a mission to infiltrate a secret lair that has something to do with both the Greek and Marvel worlds.
Another interesting pairing could be Ben and Captain America. Cap's leadership and moral compass could influence Ben, while Ben's diverse powers could be a great asset to Cap in battle. They could be in a situation where they have to fight against a mind - controlled army, and Ben's aliens like Big Chill could freeze the enemies in place while Cap gives strategic commands.
Definitely Hope Mikaelson and Loki. Loki is a master of trickery and magic in his own right. Pairing him with Hope, who has her own brand of powerful magic, would be really interesting. They could have this complex relationship where they are both trying to outwit each other at first, but then realize they need to work together to face a greater threat, like an interdimensional demon that is causing chaos in both their worlds.
Batman and Michonne would be an interesting pair. Michonne is a badass with her katana, and Batman is a master of combat. They could exchange fighting techniques and strategize against the walkers together. Their different backgrounds would make for an interesting dynamic.
Harley Quinn and Deadpool would be an extremely unexpected pair. Harley's chaotic and unpredictable nature combined with Deadpool's fourth - wall - breaking and equally chaotic personality would make for a really wild and hilarious fanfiction. They could get into all sorts of misadventures while trying to rob a bank or take on a super - villain just for the fun of it.