One possible plotline could be Harry and Neville teaming up to discover a new spell or magical ability that has been long forgotten in Hogwarts. They might explore the Forbidden Forest together in search of ingredients for a potion related to this discovery.
Perhaps there's a plot where they form a secret club within Hogwarts to help out the younger students who are being bullied. In this fanfiction, they could use their combined skills and knowledge to outwit the bullies and create a safe space for the first - years.
A really interesting plot could be that Harry and Neville find a mysterious object in the Room of Requirement. This object might be linked to the founders of Hogwarts, and they have to go on a quest to figure out its purpose and how it can be used to save Hogwarts from an impending danger.
In some fanfictions, their twin status might change the way the Sorting Hat sorts them. Instead of Harry going to Gryffindor, perhaps they both end up in Hufflepuff because of their shared values that are more in line with that house. This could lead to a completely different story arc where they are known for their loyalty and hard work rather than Harry's typical Gryffindor - like heroics. Also, their family relationships would be different. The Dursleys might have to deal with two magical children instead of one, which could change the way Harry (and Neville) grow up and view the Muggle world.
In some fanfiction, they could be on a quest to find a hidden magical artifact that was once owned by a great wizard. Along the way, they encounter various magical creatures and overcome obstacles using their combined skills and courage. Neville's knowledge of Herbology and Harry's combat skills would make them a great team. For example, Neville could use his knowledge of plants to create barriers or distractions while Harry battles any dark wizards or dangerous creatures that try to stop them.
One interesting plotline could be how Harry and Blaise's friendship develops over time. Maybe they start off as rivals in school, but as they get to know each other better, they find common interests like a love for magical creatures. This could lead to them working together on projects related to it.
One interesting plotline could be Neville, who is already good at Herbology, discovers a new way to use plants in potions with Harry's help. This changes the way potions are made at Hogwarts.
This fanfiction might explore a scenario where Harry and Neville are 'sub mates', which could imply they are in a sort of subordinate or supportive relationship to one another. It could involve them facing challenges together within the wizarding world, like dealing with dark magic threats or school problems, with their relationship being a central aspect of the story.
One possible plotline could be the struggle of acceptance. Harry Potter might face opposition from the wizarding community when he mates with a werewolf. Another could be the discovery of new magical abilities or powers that come from this unique union.
Well, perhaps in the fanfiction, the pregnancy could be a result of some sort of magical accident. For example, a misfired spell that has unintended consequences. Then, Neville and fem Harry would have to deal with not only the physical changes but also the emotional turmoil. They might seek help from their friends like Hermione and Ron, who could offer different perspectives on how to handle this situation.
Another interesting plot is when they are both part of a magical competition. They start as rivals but end up learning from each other. In one particular fanfiction, they are in a dueling tournament. Through the duels and the preparation for them, they see each other in a new light and form an unexpected bond.
One interesting plotline could be Snape and Harry having to work together on a secret project for Dumbledore. They start off distrusting each other but as they face challenges, they gradually become friends. For example, they might be researching a powerful dark magic spell that Voldemort is after, and in the process, they learn about each other's past traumas and start to empathize.
Perhaps a plotline where they go on adventures together. They could explore hidden parts of Hogwarts that no one else has found. Their different personalities - Harry's goodness and Tom's initially hidden darkness - could create a lot of tension and character development during these adventures.