One possible plot could be that the lawyer helps Harry deal with the legal aftermath of the Battle of Hogwarts. There might be issues regarding the rebuilding of the wizarding world, and the lawyer could ensure that Harry's rights and the rights of his friends are protected during this process.
Yes, there are likely 'Harry Potter has a lawyer' fanfictions out there. However, I can't specifically recommend any as there are countless fanfictions and it's hard to single out ones with this exact theme. You can try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own using relevant keywords.
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.
Hermione Granger could be a common character. Given her intelligence and knowledge of the wizarding world laws (she's always reading), she could easily be imagined as a brilliant lawyer. She would use her quick thinking and vast magical knowledge to build strong cases.
Hermione Granger could be a great candidate. She is extremely intelligent, has a deep understanding of the wizarding laws (from all her studies), and is very determined. She could use her knowledge to fight for justice as a private lawyer. Her Muggle - born background might also give her an edge in dealing with situations that require a different perspective from the traditional wizarding view.
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.