Maybe they would talk about Voldemort. Since Nagini was Voldemort's pet and weapon, Harry might ask her about Voldemort's plans or weaknesses.
Another topic could be about the other Horcruxes. Nagini was a Horcrux herself. Harry might want to know how she felt about being one. Did she have any awareness of it? And did she know about the other Horcruxes? Maybe she could provide some clues to Harry on how to destroy them more easily.
They could discuss the magical world in general. Nagini has a unique perspective as a magical snake, and Harry could be interested in things like how she experiences magic compared to humans. For example, how she senses danger or other magical creatures in the Forbidden Forest. It could also be about the origin of her snake - like nature, whether it was a curse or some other form of magic that made her what she is.
It could have Nagini in a more human - like form at times, having conversations with other characters. The lemon aspect might be in the form of a romantic or sensual undertone between Nagini and another character, like a Death Eater. For example, they might share some intense glances during a meeting at Voldemort's lair.
There could be a situation where Nagini is in a weakened state for some reason. Harry, being the hero he is, might face an internal struggle. On one hand, he knows Nagini is Voldemort's pet and has been involved in many evil deeds. But on the other hand, he might see her as a victim of Voldemort's dark magic. So he could either try to kill her or try to free her from Voldemort's control.
One reason could be a change of heart. Maybe Nagini realizes that Voldemort's cause is wrong and sees something in Harry that makes her want to protect him, like his kindness or his will to fight for good.
It could also be about Harry's future. Snape might be giving advice on how to deal with Voldemort. He could be sharing his insights on the Dark Arts and how Harry can use that knowledge to his advantage without being corrupted by it. Snape has a complex understanding of the Dark Arts and might be trying to prepare Harry for the battles ahead.
Well, he might be really confused at first. Fanfiction often takes the original story in different and sometimes wild directions. He could be shocked to see how some fans have imagined his relationships or new adventures that are quite different from his own experiences.
Well, it could be that the dragons are in danger due to some dark magic. Harry talks to them to figure out a way to save them. In the process, he learns more about the magical world of dragons and their history. He might even find out that the dragons have a connection to the founders of Hogwarts.
They might talk about their old classmates who couldn't make it to the reunion.
In many Harry Potter and Nagini fanfictions, the relationship between them is often explored in unique ways. Some might depict Nagini as having a more complex personality than in the original books. For example, she could be shown as a victim of dark magic rather than just a tool of Voldemort.
It could still be Voldemort. Even though the focus is on Harry and Nagini, Voldemort is Nagini's master and the main source of evil in the Harry Potter world. He would likely try to stop Harry from getting too close to Nagini or from finding out any secrets Nagini might hold.
One common theme is Nagini's humanity. Many fanfictions try to give her more depth and show that she has feelings and thoughts beyond being just a tool for Voldemort.