One idea could be that Rosalie and Jacob are forced to work together on a mission to protect the town from a new supernatural threat. They start off hating each other but gradually come to respect one another's skills.
I really like 'A New Dawn for Jacob and Rosalie' in the world of jacob black and rosalie hale fanfiction. It starts with a very interesting premise where Jacob and Rosalie are forced to work together on a mission that could save their respective clans. As they spend more time together, their relationship evolves in a very engaging way. There's also 'Moonlit Meetings' which is full of intense emotions and great character development for both of them.
A fanfiction could be set in an alternate universe where Jacob and Rosalie are both students in a prestigious art school. Jacob is into more rugged and earthy forms of art, while Rosalie is drawn to the refined and classical styles. Their clash of artistic visions at first leads to rivalry, but eventually they inspire each other to create new and unique art forms.
You can start by looking on fanfiction.net. It has a large collection of fanfictions, and you're likely to find many stories about Jacob Black and Rosalie Hale there. Another good place is Archive of Our Own (AO3). It has a very user - friendly interface and a wide range of fan - created content, including some great pieces involving these two characters.
They could start by having common goals. For example, both might be interested in protecting a certain human from the darker elements of the supernatural world. This shared objective can bring them together and make them interact more, gradually building a relationship.
One idea could be that Hermione is on a research mission in the forest where Jacob Black's pack roams. Their paths cross and at first they're wary of each other. But as they start to communicate, they realize they can help each other. Jacob can provide Hermione with knowledge about the forest's magic that she didn't know, and Hermione can offer her magical expertise to solve some problems the pack is facing, like a mysterious dark magic that's affecting the forest animals.
One plot idea could be that Rosalie somehow ends up in the Harry Potter universe. She might initially be a bit of an outcast due to her vampiric nature but Harry is intrigued by her. They gradually form a bond as they face some common enemies.
One idea could be that Percy and Jacob meet in a strange dimension where the rules of their respective worlds are all jumbled up. They have to team up to find a way back to their own realms while facing challenges specific to both their mythologies, like Percy dealing with a werewolf - themed water monster and Jacob having to navigate through a maze filled with Greek demigods' traps.
One idea could be Jacob and Renesmee going on a journey to discover more about Renesmee's unique abilities. They could encounter other supernatural beings along the way who either help or hinder them.
One interesting plotline could be Rosalie and Bella becoming best friends and helping each other out in the vampire - human world. For example, Rosalie could teach Bella how to better defend herself against other vampires using her own unique skills. Another plotline might be them teaming up to change some of the rules in the Cullen family. Maybe they work together to make it easier for Bella to be more involved in vampire matters without always being in danger.
One interesting fanfiction could be about Jacob and Rosalie forming an unlikely alliance to protect the Cullen family from a new threat. They put aside their differences and use their unique abilities to fend off the danger.