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.
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.
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.
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 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.
First, consider the world of One Piece. The OC should have a backstory that fits within the pirate - filled universe. For example, if they grew up on an island known for its unique fighting style, they could have skills related to that. Second, give the OC a personality that would either complement or clash with Law's. If Law is more brooding and calculating, the OC could be more light - hearted and impulsive, creating an interesting dynamic.
I really liked the fanfic 'A Chance Encounter with Law'. In this, the OC is a doctor in training, which creates an interesting dynamic with Law. It's full of medical jargon and their relationship progresses in a very natural way.
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.
First, understand Law's character well. He's complex, so research his backstory, his goals in One Piece. For the OC, create a character that can fit into the One Piece world. For example, give them a relevant skill or a backstory related to the Grand Line.
One plot idea could be the Oc and Hermione having to navigate the complex rules of the marriage law. Maybe they initially resist but gradually find common ground and unexpected feelings for each other. For example, they could be paired together by the law and at first be at odds, but as they face challenges in the wizarding world together, they start to rely on and care for one another.