One of the most common transformations for Harry in fanfic is into his Patronus form, which is a stag. This is because it's a very recognizable and powerful part of his character in the original series. Another common one is a werewolf - like transformation, often in stories that explore darker or more alternative storylines.
Fanfic writers sometimes create entirely new transformation concepts for Naruto. These could be inspired by different elements or even crossovers with other fictional universes. A transformation might be linked to a new power source Naruto discovers, like an ancient chakra crystal that changes his form and abilities. It adds a whole new layer of creativity to the Naruto universe.
One popular transformation in Naruto fanfics is the 'Celestial Nine - Tails' form. In this, Naruto's Nine - Tails chakra is enhanced with celestial energy, giving him a more god - like appearance with glowing blue - white fur and enhanced speed and strength. It's often used in fanfics where Naruto faces extremely powerful foes.
Survival is likely a big theme. In Starship Troopers, the soldiers are constantly fighting for survival against the bugs. In Harry Potter, the characters are also in a fight for survival against Voldemort and his Death Eaters. So in fanfics, there could be a merging of these survival situations, like the wizards having to survive in the bug - infested worlds of Starship Troopers.
Often, Harry uses his intelligence and courage. He could find loopholes in Umbridge's rules and use them against her. For example, if she made a rule that was against the school's original code, Harry could prove it and get her in trouble. Another way is that he could get the support of the magical creatures at Hogwarts. If the centaurs or the house - elves had a problem with Umbridge, Harry could use that to his advantage to get her fired.
Voldemort is quite common. Since he's the main villain in the original series, fanfic writers often use him to create darker storylines. Bellatrix Lestrange is also popular. Her insane and violent nature gives a lot of room for dark interpretations.
Common themes in such stories include the transformation of identity. A person may change into something or someone else, which forces them to re - evaluate who they are. There can also be a transformation of the setting that is related to the erotic aspect. For instance, a normal place might transform into a more sensual or mysterious location, enhancing the overall erotic atmosphere of the story.
In some Harry Potter fanfics, flying is used for exploration. Characters fly over the Forbidden Forest to discover its secrets or fly to uncharted islands. They might encounter magical creatures during these flights. Also, flying can be part of a character's escape plan. When facing an enemy or a difficult situation at Hogwarts, flying away on a broomstick can be a quick way out.
Obviously, Harry Potter himself is very common. He's the central character of the original series, so many fanfics still revolve around him. Hermione Granger is also frequently seen. Her intelligence and strong - willed nature make her a great character to explore in different scenarios. Ron Weasley is another common one, often as part of the trio with Harry and Hermione.
Hermione Granger is quite common. Given her intelligence and strong - willed nature, she often plays a significant role in dealing with marriage law issues. Harry Potter himself also frequently appears as he is a central figure in the wizarding world. Draco Malfoy is another one. His pure - blood status and family's influence make him a key character when it comes to marriage laws that often involve pure - blood traditions.
One common theme is identity. When a female character transforms into an animal, she often has to re - evaluate who she is. For example, in stories where a girl becomes a wolf, she may struggle with her newfound predatory instincts compared to her former human self.