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.
Perhaps a plot where Draco and Neville are both caught in a magical mystery outside of Hogwarts. They have to rely on each other to survive. Draco's Slytherin cunning helps them avoid some dangerous traps, while Neville's Gryffindor courage gives them the strength to face the unknown threats head - on. As they go through this adventure, they develop a bond that surprises their friends back at school, and they end up changing the views of their respective houses about the other.
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 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.
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.
One possible plotline could be Neville struggling to hide her true gender identity while dealing with the challenges at Hogwarts. Maybe she has to constantly come up with excuses for not participating in certain all - boy activities.
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.
Well, in some 'Draco turned into a ferret fanfiction', there could be a plot where Draco in ferret form discovers new things about the Hogwarts grounds that he never noticed as a human. Maybe he stumbles upon a secret passage or a hidden magical creature. Also, there could be a story where his transformation is a result of a misfired spell by a student, and the whole school has to figure out how to reverse it.
A third plotline could be Draco's struggle with his identity. He has the memories of his previous life, but he's now in Draco's body. He has to deal with Draco's family expectations, his reputation at Hogwarts, and at the same time, he wants to do things differently. This internal conflict could lead to a lot of character development and new story arcs.
One interesting plotline could be that they discover their twin relationship early on and try to keep it a secret from everyone at Hogwarts. They might secretly help each other out in classes and with their rivalries against other houses.