A human can be a helper. They could assist the mouse detectives by providing resources that are difficult for mice to obtain, such as certain tools or information from the human world.
One way is to give the human a unique backstory. For example, if it's a detective in the Great Mouse Detective world, he could have a past that led him to be interested in solving cases in both the human and mouse worlds. Maybe he grew up near a place where the mice and humans interacted a lot, so he has a natural curiosity.
Obviously, the great mouse detective himself is very popular. His intelligence and charm draw readers in.
One interesting story could be about the great mouse detective solving a mystery in an old, abandoned cheese factory. The clues are hidden among the cheese wheels and old machinery.
A modern human can also be a learner. They can be in awe of the magic and the rich history of Azeroth. Their journey of learning about this new world can be the focus of the fanfiction. They might start as a novice but gradually become an important figure by using their modern - day knowledge in combination with the new knowledge of Azeroth.
A human could be a healer. Given that Middle - Earth has its own forms of medicine, the human could bring in different approaches. They might use herbal remedies from their own knowledge combined with the local plants. This could be especially useful in treating wounds after battles or during plagues.
Definitely Sherlock Holmes. Since The Great Mouse Detective is inspired by Sherlock Holmes, a crossover with the original Holmes would be very interesting. They could compare and contrast their detective methods.
The OC could be from a different part of the mouse world with unique skills, like being able to communicate with certain insects that can help in solving crimes. For example, they could get information from a spy spider about a villain's lair location. The OC then teams up with the great mouse detective to use this new lead to solve a big case.
One idea could be a crossover with Sherlock Holmes in the human world. The great mouse detective could team up with Holmes to solve a mystery that spans both the mouse and human realms. Maybe there's a villain causing chaos in both worlds and their combined skills are needed.
Probably courage and problem - solving. The mouse detective must be brave to face the challenges and mysteries in the story. And he has to use his intelligence to solve the problems he encounters. Also, there could be a theme of community, where different mouse characters work together towards a common goal.
The human could be a villain. He might be jealous of the fame and power of the Kung Fu heroes in the Valley of Peace. He uses his own human - based fighting skills, which could be more brutal and unorthodox compared to the Kung Fu of the valley, to cause trouble and try to take over the place. For example, he might use weapons that are not commonly used in the valley, like a long - range crossbow or a spiked mace.