One interesting idea could be the nature of Voldemort's pet. Maybe it has some unique magical abilities that are different from ordinary magical creatures. For example, it could have the power to communicate telepathically with Voldemort.
One idea could be a fanfiction where Voldemort is redeemed. Maybe after a near - death experience, he has a change of heart and tries to make amends for all the evil he has done. He could start by secretly helping Harry Potter against some other dark forces that emerge in the wizarding world.
Another interesting plot could be a backstory about how Snape first got involved with Voldemort. Maybe it was out of a thirst for power initially, but then his views changed over time. This could be delved into in great detail, showing his transformation from a potential dark wizard follower to a hero in the fight against Voldemort.
Maybe Harry discovers that Voldemort has a hidden motive for taking him as an apprentice. It could be that Voldemort believes Harry has a power that can be harnessed for a greater, more complex plan that involves not just ruling the wizarding world but changing the very fabric of magic itself. Harry then has to figure out how to either use this to his advantage or stop Voldemort from achieving his goal.
Perhaps Voldemort's son is raised in secret by a group of witches and wizards who want to give him a chance at a normal life. But as he grows older, he starts to show signs of his father's power. This could lead to a conflict within him as he decides whether to embrace or reject his dark powers. He could also go on a journey to learn about his father's past, only to be horrified by what he discovers and then vow to be different.
One idea could be a story where a cat discovers it has the power to talk to other animals and they all team up to save their neighborhood from a construction project that would destroy their habitats. Another could be a dog that gets lost in a magical forest and has to find its way home while befriending strange forest creatures.
Perhaps there's a fanfiction where Hermione disguises herself as one of Voldemort's supporters to infiltrate their ranks. She has to be extremely careful not to be discovered as she tries to find a way to stop a major plot they are hatching. Along the way, she discovers the complex reasons some people became supporters and uses that knowledge to her advantage.
Another plot could be that they are married in an alternate universe where their backstories are different. Maybe Voldemort was never as consumed by dark magic from the start. Harry and Voldemort could then discover that they have a deeper connection than they thought, and their marriage could be a journey of self - discovery for both of them, as they explore their relationship in a world that is both familiar and yet very different from the one they knew in the original story.
One idea could be that Harry starts having strange dreams where he sees through Voldemort's eyes. This could lead to him unknowingly helping Voldemort in some small ways at first, like revealing the location of some minor Order of the Phoenix safe houses. Another plot could involve the other characters slowly realizing something is wrong with Harry. For example, Hermione might notice that Harry is using some Dark Magic spells without hesitation, which is completely out of character for him.
In some fanfiction, there could be a new kind of pet that is created from the combined magic of Harry and Voldemort, perhaps accidentally. This pet has powers that are both light and dark. It could be a small creature like a mouse that has the ability to travel between the realms of the living and the dead. Harry and Voldemort then have to compete to gain the loyalty of this unique pet which holds great power.
Well, a plot could be that Voldemort has a change of heart after falling in love with Ginny. He starts to question his dark ways and Ginny, seeing this potential for good in him, decides to marry him in the hopes of fully turning him around. But this is met with great resistance from the wizarding community who see Voldemort as irredeemable. Ginny then has to fight not only for Voldemort's redemption but also for their relationship to be accepted.