A plot could involve Law being transported to a post - apocalyptic world. There, he meets a group of survivors. Law uses his knowledge and skills to help them, but also discovers that there are some strange things going on. There are mutated creatures that seem to be related to the experiments he once saw in the One Piece world. He has to figure out how to stop these mutations from spreading further and also find a way back home.
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.
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.
Well, if Law is crossed over with a character like Sherlock Holmes, for example. Law's calm and calculating nature can be paired well with Holmes' deductive skills. Law's more secretive side can also add an air of mystery, much like Holmes. They could have a sort of rivalry at first, each trying to outwit the other in solving a case, but then come to respect each other's abilities.
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 idea could be that the Akatsuki from Naruto are somehow connected to the Noah family in D Gray - Man. Maybe they are both after a powerful and ancient artifact that has the power to change both worlds. Another plot could involve Naruto characters having to deal with Innocence and Akuma in the D Gray - Man universe. For example, Naruto's chakra could be seen as a similar power source to Innocence and he has to learn how to use it to fight the Akuma.
One idea could be that the characters from Blue Exorcist are transported to the world of D Gray - Man. For example, Rin Okumura could meet Allen Walker. They might team up to fight a common enemy that is a mix of the demons from Blue Exorcist and the Akuma from D Gray - Man. Another plot could involve the exorcism techniques from both worlds being combined to defeat a super - powerful evil force.
One plot idea could be that the characters from D Gray - Man are sent to the world of Black Butler to investigate a series of strange disappearances that seem to be related to a powerful Akuma - like entity. The Black Butler characters, on the other hand, mistake the D Gray - Man group for some sort of new threat. This could lead to some interesting battles and misunderstandings before they figure out they need to work together.
One idea could be the exorcists from D Gray - Man ending up in the world of Highschool DxD. They might initially be mistaken for devils or angels due to their strange powers. For example, Allen Walker's cursed eye could be seen as a very unique and perhaps threatening power. Another plot idea could involve the search for the Innocence in the Highschool DxD world. Since both worlds have different forms of power sources, it would be interesting to see how the Innocence interacts with the Sacred Gears. Maybe a character with a Sacred Gear could also be a potential host for the Innocence, creating a whole new dynamic.
One idea could be that the Exorcists from D Gray - Man end up in the Fairy Tail world due to a strange magical anomaly. They team up with the Fairy Tail wizards to fight against a common enemy that is causing chaos in both their worlds. For example, a powerful dark mage who has somehow acquired Innocence and is using it in a corrupted way.