I think Ruby and a Big Daddy could be an interesting pairing. Ruby is full of energy and curiosity. She would probably be the first to approach a Big Daddy and try to understand it. Her speed could be useful in dodging the Big Daddy's attacks while she figures out how to communicate with it or work together.
Yang and a Big Daddy would also make a great pairing. Yang is all about strength and confidence. She would likely see the Big Daddy as a worthy opponent at first. But then, realizing their potential as allies, she could use her brawler - style combat to complement the Big Daddy's brute force. They could have some epic battles side by side against powerful foes.
A RWBY and Big Daddy crossover fanfiction could be really interesting. It might involve the characters from RWBY encountering Big Daddies in some sort of alternate dimension or on a mission. For example, Ruby could be initially scared by the imposing Big Daddies but then realize they can be allies against a common enemy. Maybe they team up to fight Grimm that have been infected with some kind of strange technology similar to what powers the Big Daddies.
The Doom Slayer and Ruby. Ruby is young and full of hope, always looking for adventure. The Doom Slayer is a stoic, battle - hardened warrior. Their interaction would be really interesting. Ruby could be in awe of his strength and weapons, and the Doom Slayer might see a bit of his own unwavering will in Ruby.
One idea could be that a character from another universe gets transported into the BioShock world and has to team up with a Big Daddy. Maybe they start off as enemies, but as they face common threats like the Splicers, they form an unlikely alliance. The Big Daddy could protect the new character while they figure out how to get back to their own world.
Blake and Kirby would also be a great combination. Blake has a mysterious side to her, with her Faunus heritage and her past. Kirby's innocent and curious nature could make Blake open up more. And Kirby might be intrigued by Blake's ability to use her shadow and her skills as a Huntress. Their relationship could develop in ways that explore both their characters' depths in the RWBY x Kirby fanfiction world.
One interesting idea could be a crossover of different timelines within RWBY. For example, characters from the past meet the current ones and have to deal with the differences in their worlds. Another could be a crossover of different teams in unexpected ways, like Team RWBY having to team up with a rival team to face a common enemy that is more powerful than any they've faced before. And sometimes, there are crossovers that involve alternate versions of the characters, where their personalities or abilities are slightly different, which can lead to very unique and engaging storylines.
I think Supergirl and Black Widow would be an interesting pairing. Black Widow is a skilled spy and fighter, while Supergirl has superpowers. They could learn from each other. Black Widow could teach Supergirl some stealth techniques, and Supergirl could protect Black Widow during battles with her super strength and speed.
A really cool pairing could be Naruto and Quorra. Naruto's energetic and positive nature could contrast with Quorra's more reserved personality. And Naruto could teach her about the concept of chakra and the ninja way, while she could show him the ins and outs of the digital realm.
One popular 'big daddy crossover fanfiction' might be the one where Big Daddy is crossed over with a superhero universe. For example, in some stories, Big Daddy could be in the same world as Batman and have to deal with Gotham's crime. It's interesting to see how his unique abilities would interact with the already established characters and settings in the Batman universe.
A really interesting pairing could be Edward from Twilight and Evy from 'The Mummy'. Edward has his vampire abilities and is very protective, while Evy is smart and brave. They could work together to solve some mystery related to both their worlds. Maybe Edward could use his super speed to help Evy escape from some mummy - related danger, and Evy could use her knowledge of ancient history to help Edward understand more about the mummy's origin.
I think Spiderman and Batman would be a really interesting pairing. They both rely on their wits and gadgets. Batman could offer Spiderman some more high - tech equipment and in return, Spiderman could show Batman some new acrobatic moves.