Well, in non - inappropriate 'Harry Potter x Moaning Myrtle' fanfiction, there could be a theme of mystery. Myrtle has a lot of knowledge about the castle's past and secrets, and Harry is always interested in solving mysteries. So they could team up to uncover something about Hogwarts' history. Another theme could be redemption. Myrtle might find a way to redeem herself or be seen in a new light through her interactions with Harry.
A possible theme is exploration. They could explore the hidden parts of Hogwarts together, since Myrtle is often associated with the bathrooms and other less - visited areas. Also, there could be a theme of acceptance. Harry, being the kind - hearted character he is, might accept Myrtle for who she is in a way that no one else has, and this could lead to a unique bond between them.
One common theme could be friendship. Despite the odd pairing, they could form an unusual friendship. Maybe Myrtle helps Harry in some way that others can't, like providing insights into Hogwarts' secrets. Another theme might be growth. Harry could help Myrtle move past her sad state and grow as a character.