A crossover with 'Bleach' could be really cool. Law could end up in the Soul Society and his powers would be a mystery to the Shinigami. His ability to create 'Rooms' and manipulate objects within them could be seen as a threat or a new form of spiritual power. He might also get involved in the battles between the Soul Reapers and the Hollows, bringing his own brand of justice and strategy.
Maybe a crossover where Law ends up in a world full of magic. He has to use his medical skills and swordsmanship to survive. He meets a powerful mage who is intrigued by his powers. Together, they go on a quest to stop an evil force that is threatening to destroy both their worlds. Law's ability to create rooms could be really interesting in a magical setting, like using it to contain dangerous spells.
One idea could be that the OC is a healer who joins Law's crew. They start off butting heads because Law is so used to being in control, but gradually he comes to rely on the OC's unique healing abilities and they develop a deep bond.
One idea could be a story where Law and Doflamingo are forced to team up due to some external threat to the One Piece world. They initially clash because of their very different moral compasses but gradually come to understand each other's motives. Another plot could involve a time - travel scenario where Law goes back in time and has to deal with a younger Doflamingo, trying to change the course of events that led to Doflamingo's tyranny. Or perhaps a story where they are both stranded on a mysterious island and have to rely on each other's skills to survive, all while Law is constantly tempted to take Doflamingo down.
One idea could be Law somehow getting transported to the RWBY universe. He could team up with Team RWBY to fight against a common enemy, like a new type of Grimm that has some sort of devil fruit - like abilities. Another plot could involve Law using his Ope - Ope no Mi powers to heal some of the seriously injured characters in RWBY, which would lead to interesting interactions as they try to understand his powers. A third plot idea is that Law gets caught in the middle of the conflict between the different kingdoms in RWBY and has to use his strategic mind to navigate through the politics while also dealing with the threat of the Grimm.
One interesting Trafalgar Law OC fanfiction could be one where the OC is a doctor on his ship, adding a new dynamic to the medical team. They might have a unique backstory that ties into Law's past in some way.
In fanfiction, Trafalgar Law is often depicted as highly intelligent. Writers usually play up his surgical skills and strategic mind, making him a key figure in complex plotlines. For example, in many fanfictions, he's the one who comes up with elaborate plans to outwit the enemies.
Well, a great plot could be that in a fanfiction, Law and Nami accidentally swap their abilities. Nami has Law's Ope - Ope no Mi powers and Law has Nami's Clima - Tact skills. They then have to learn how to use these new powers while also dealing with the chaos it causes on the ship. Also, perhaps there's a story where Nami gets sick and Law is the only one who can cure her, but he needs to find a very rare ingredient on an enemy - occupied island. Another one could be that they are on a mission together to steal a valuable jewel from a heavily guarded fortress and they plan an elaborate heist.
First, decide which universe you want to cross Law with. Then, think about how Law would fit into that world. For example, if it's a 'Harry Potter' crossover, consider how his powers would interact with magic. Next, come up with a basic plot. It could be something like Law is on a mission in the wizarding world to find a powerful magical artifact. Then start writing an engaging opening, like Law suddenly appearing in Diagon Alley and causing a stir.
The contrast between their personalities makes it interesting. Law is cool, calm, and calculating, while Baby 5 is more impulsive and needy. In fanfiction, this contrast can lead to all sorts of interesting interactions, from humorous banter to deep emotional connections.
One popular Trafalgar Law fanfiction could be those that explore his backstory in more depth. For example, a fanfic might focus on his time in the North Blue before joining the Shichibukai. Another type could be adventure - themed ones where Law teams up with other characters from 'One Piece' in new and exciting quests.