Well, a very common theme is the exploration of the past. Through flashbacks and the process of regaining memories, the fanfiction often delves into Ichigo's past. This could include his relationship with his family, his training, and his battles. And there's also the theme of hope. Despite the amnesia, there's always the hope that Ichigo will get his memories back and return to his normal self.
The theme of identity crisis is often present. Ichigo may not recognize his powers or his role in the Soul Society. For example, in some fanfictions, he might not understand why he has a Zanpakuto. Also, the theme of redemption can be there. Maybe he made some mistakes in the past that he can now correct without the burden of his old memories.
One common theme is self - discovery. Since Ichigo has lost his memories, he has to figure out who he is again. Another theme is the importance of relationships. His friends play a crucial role in helping him regain his memories or adapt to his new state.
Friendship is a very common theme. Her friends usually come together to help her, like in many fanfics where they use their unique skills to try and jog her memory. For example, Rainbow Dash might show her some of their flying adventures together.
One 'ichigo gets amnesia fanfiction' I know is 'Memory's Lost Path'. It shows Ichigo struggling to regain his memories while his friends try to help him. It has a really interesting take on how his relationships change due to the amnesia.
A common plot is the struggle of his friends to make Hiccup remember who he is. They might show him his old inventions or take him to places that hold special memories. Another plot is Hiccup having to relearn dragon training from scratch, which is interesting as he has to rediscover his own skills. And often, there's a plotline about his relationship with Astrid being put to the test since he doesn't remember their history.
A common theme is the exploration of their dual natures. Since Ichigo has his own powers and the female hollow ichigo has her unique hollow characteristics, fanfictions often play with how these two different yet connected aspects interact.
One common theme is the battle between good and evil within Ichigo. His normal self represents the good, while Dark Ichigo represents the more primal and sometimes malevolent side. Another theme is self - discovery. As Ichigo grapples with Dark Ichigo, he often discovers new aspects of his own power and personality.
In this type of fanfiction, a plotline could be that Ichigo's sickness is caused by some sort of spiritual poison or curse. His friends then have to go on a quest to find the antidote. It could involve a lot of detective - like work, like figuring out who poisoned him and why. Another aspect could be that Ichigo's sick state makes him more vulnerable to his inner demons. He might have hallucinations of past failures or enemies, and he has to fight them off mentally while also dealing with his physical illness.
Well, a theme might be the exploration of trauma. Ichigo's experience of being abused would surely cause him a great deal of trauma, and the fanfiction could delve into how he copes with it. It could also be about the discovery of inner strength. Despite the abuse, Ichigo could find strength within himself that he didn't know he had. And perhaps a theme of isolation, as he may be pushed away from others due to the abuse.
One common theme is identity. When Bella has amnesia, she often struggles to figure out who she is, especially in relation to her relationships with others like Edward or Jacob. Another theme is rediscovery. She has to rediscover her past, her feelings, and her place in the world she once knew.
In Naruto amnesia fanfiction, the theme of redemption can be seen sometimes. Maybe Naruto did something in the past that he now doesn't remember, and his amnesia gives him a chance to start anew and make up for it. Another common theme is the exploration of the past. His friends might dig into Naruto's past to help him remember, which uncovers secrets and forgotten events.
Another theme is friendship. Given that both characters are known for their strong bonds with their friends in their original series, fanfictions like to explore how they would form a friendship with each other. They might start off as rivals and then grow to respect and trust one another over time.