A wizard from a far - off land who comes to Hogwarts as an exchange student. He could have a whole new set of magical abilities that are different from what the Hogwarts students are used to, like being able to control the weather in a very specific way. This would add an interesting dynamic to the story as Harry and his friends learn from him.
One idea could be to focus on Harry's relationship with Sirius Black in a more in - depth way. Maybe Harry discovers some hidden secrets about Sirius that even Sirius himself doesn't know yet. Another could be to create a new magical creature that is somehow related to the events of the third year, like a creature that is connected to the time - turner.
A common plot could be their exploration of the Hogwarts grounds using the Marauder's Map. They might stumble upon some hidden passages or secrets that are related to the events of that year. Another one is dealing with the threat of Sirius Black. Hermione could be researching ways to protect Harry while Harry is more impulsive and wants to face the danger head - on. Also, there could be a sub - plot about their academic progress, like Hermione acing all her exams and trying to help Harry pass his, especially in subjects like Potions.
It could also be a dark wizard from the past who has been trapped in a hidden part of Hogwarts and is now released during Harry's third year. This wizard could have a vendetta against the Potter family or Gryffindor house and start causing trouble.
A really cool plot idea would be if there was a new magical creature introduced in Harry's third year that was related to the Dementors in some way. Harry could have to learn about this creature to better understand how to combat the Dementors and it could add a whole new layer to the story.
One possibility is that the fanfic delves into Hermione's over - studying for their third - year classes and how Harry tries to get her to relax a bit. They might have an adventure in Hogsmeade that goes wrong when they encounter some dark magic remnants from Sirius Black's escape. Harry could use his skills on the Quidditch pitch to boost his confidence, and Hermione would be there cheering him on while also secretly worrying about the lurking danger around them.
A great plot twist could be that the Dementors are being controlled by someone other than the Ministry. They start attacking students more frequently and specifically target Harry because they are being used to find something that Harry unknowingly has. This would make the already dangerous situation in the third year even more intense.
One great 'Harry Potter' six - year fanfic is 'The Sixth Year Adventures'. It follows Harry, Hermione, and Ron as they face new challenges in their sixth year at Hogwarts. The story has a great plot that stays true to the magical world created by J.K. Rowling. It includes exciting new spells and interesting interactions between the characters.
A half - vampire student. This character could have unique abilities like super speed and enhanced senses. They could be a bit of an outcast, but also have a mysterious allure and help Harry in unexpected ways, perhaps in dealing with the Dementors.
Well, I've also read 'Harry and Hermione: Third - Year Escapades'. In this one, they embark on an unexpected journey outside of Hogwarts due to a prophecy they discover. Their third - year knowledge and skills are put to the test as they encounter various magical creatures and obstacles along the way.