Well, perhaps Harry Potter, in the guise of Loki, uses his trickery skills in the wizarding legal system. He could be defending those wrongly accused of using dark magic, like using his shape - shifting abilities to find evidence that others can't.
He would probably interact with Hermione in an interesting way. Hermione, being a stickler for the rules, might be both impressed and a bit frustrated with Harry's Loki - like ways of bending the law. He could use his charm to convince her that his unorthodox methods are necessary to achieve justice.
Well, in the world of fanfiction, just about any concept can be explored, so it's highly likely that there is a 'harry potter is loki reincarnated' fanfiction. Writers love to mash up different universes and characters. This particular concept could bring in elements of Loki's mischievous nature into Harry's character, creating a whole new set of adventures and challenges for him in the Hogwarts setting.
Loki is a powerful Asgardian with abilities that are more in the realm of the supernatural in a different way than Harry's magic. But Harry has a strong will and has faced some extremely tough opponents. In a fanfiction, if the writer decides to give Harry access to some of the most powerful and ancient magic in the wizarding world, he could be more powerful. For example, if he could use the magic that created the Hogwarts itself, he might be able to stop Loki quite effectively. Also, Harry has the support of his friends and the entire wizarding community behind him, which could give him an edge in power.
It's hard to say. Loki has a wide range of magical abilities from Asgard. He can shape - shift, create illusions, and has a certain level of telekinesis. However, Harry Potter has the power of love and friendship on his side, which has proven to be a very strong force in his world. Also, Harry is very skilled at defensive and offensive magic within the wizarding world. So it really depends on the situation.
There could be an entirely new goblin character created just for this fanfiction. This goblin lawyer could be an outcast from the goblin community because of his more progressive views on wizard - goblin relations. Harry, seeing his potential, hires him to help with his legal battles that require a fresh perspective.
It could be Hermione. She's always been very book - smart and she could have decided to study law in the wizarding world. She already has a great understanding of the rules and regulations at Hogwarts, so it would be a natural progression for her to become a lawyer. And she would be very dedicated to helping Harry with any legal issues.
Luna Lovegood might be an unexpected but interesting choice. She has a unique perspective on the wizarding world. While she may seem a bit eccentric, her ability to think outside the box could be useful in a courtroom. She could come up with creative defenses for Harry, and her calm and dreamy nature might be able to sway a jury or judge in Harry's favor.
Another theme is justice. Harry, being the hero we know from the original series, is always striving for justice in these fanfictions. He uses his legal skills to right wrongs and protect the innocent in the wizarding world. For example, in many of these stories, he defends those who are wrongly accused of using dark magic.
One possible plotline could be Harry using his knowledge of the wizarding laws, which he learned through his years at Hogwarts and his experiences with the Ministry of Magic, to defend wrongly accused wizards in a wizarding court. Another could be him trying to reform the wizarding legal system which he might find full of loopholes and biases.