In 'Harry is a Basilisk' fanfiction, the theme of transformation is quite common. Harry's physical transformation into a basilisk brings about a whole new set of challenges. There's also the theme of loyalty. His friends' loyalty to him may be tested when they find out he's a basilisk. Additionally, the theme of mystery can be present as there are many unknowns about being a basilisk in the context of the Harry Potter universe.
One common theme is the idea of Harry overcoming his fear of the Basilisk. Another is the discovery of a hidden power or connection related to the Basilisk. For example, in some fanfics, Harry might find out he has a special ability to communicate with the Basilisk.
One common theme is redemption. Many fanfics explore the idea of the basilisk having a chance to atone for its actions, like in 'A Basilisk's Redemption'. Another theme is mystery. Fanfics often try to uncover more about the basilisk's origin and its true nature, as seen in 'The Secret of the Basilisk'.
One common theme is the struggle against prejudice. Since Slytherin has a reputation for being the 'evil' house, Harry often has to fight against the preconceived notions of others. For example, in many fanfictions, the Gryffindors are initially suspicious of Harry being in Slytherin.
Another theme is loyalty. He has to decide who he is loyal to - his house, his friends, or his own beliefs. In some fanfictions, he might face a dilemma where his loyalty to Slytherin clashes with his friendship with students from other houses. This creates a lot of interesting plot twists.
One common theme is the struggle for acceptance. Since Slytherin is often seen as the more 'ambitious' and sometimes 'dark - leaning' house in the Harry Potter universe, a pregnant Slytherin might face more judgment. So, the character often has to fight for acceptance from her peers and family.
In 'Harry is a Slytherin' fanfics, a frequently seen theme is the re - evaluation of Hogwarts houses. Harry's presence in Slytherin makes people question the stereotypes. Family heritage can be a theme too, as Slytherin has a certain reputation related to pure - blood families. Another theme is the forging of new loyalties. Harry has to decide who to trust within Slytherin, which is different from his experiences in Gryffindor. And of course, the theme of personal growth as Harry develops in an environment that is initially foreign to him.
Another theme is friendship. Harry forms new friendships in Slytherin that he might not have had in Gryffindor. These friendships often cross the traditional house boundaries and are quite interesting to read about. For example, he might become friends with some of the more complex Slytherin characters who have their own motives and backstories. Also, the theme of self - discovery is prominent. Harry discovers new aspects of his magic and personality as he is in a different house environment.
Another theme is loyalty. Despite the reputation of Slytherin sometimes being seen as self - serving, fanfics often show the OC's loyalty to their friends or house mates. For example, they might stand up for a fellow Slytherin in a difficult situation. It also shows that loyalty can be complex and not always straightforward. Moreover, redemption is a theme too. Some Slytherin OCs start off with a not - so - great reputation but through their actions and growth, they seek redemption and to change others' perception of them.
One common theme could be friendship. Since MLP is big on friendship, it might be incorporated into how Harry and the MLP characters interact, even with the presence of the basilisk element. Another could be magic and its different forms in both universes.