Well, one theme is power acquisition. In many 'I was reincarnated' BL light novels, the protagonist, after being reincarnated, tries to gain power in the new world. This power can be magical, political, or social. And of course, the theme of love and attraction between male characters is always there. It can be a slow - burn kind of love or a more sudden realization of feelings. There's also often the theme of friendship, which sometimes blurs into something more romantic as the story progresses.
A common theme is identity discovery. When the character is reincarnated, they often have to figure out who they are in this new world. Also, there's the theme of male - male relationships development. Since it's BL, the relationship between male characters is a key part, like how they go from strangers to having deep emotional bonds. Another theme is the adaptation to a new world, dealing with new magic systems, social hierarchies, etc.
In these novels, the theme of redemption can be present. The reincarnated character might have made mistakes in their past life and is trying to make up for it in the new world while also exploring their relationships with other male characters. Another common theme is the exploration of different cultures. Since they are reincarnated into a different world, they get to experience new cultures, and this often affects their relationships, especially the BL relationships. Also, the theme of self - improvement is common as they strive to become better both for themselves and for their loved ones.
Power - building is another theme. Since they are in new and often dangerous worlds, they need to gain power. In 'Re:Zero - Starting Life in Another World', Subaru has to learn how to fight and use his Return by Death ability to become stronger over time. This theme keeps the readers engaged as they watch the characters grow.
One common theme is starting anew. The protagonist gets a second chance at life in a different world, often with some special abilities or knowledge from their previous life. For example, in many of these novels, the character uses their modern - world knowledge to their advantage in a medieval - like fantasy world.
One common theme is self - discovery. The transmigrated character often has to figure out who they are in the new world and in the context of their new relationship. For example, in many novels, they might discover new aspects of their personality through their interactions with their love interest.
Survival is a common theme. When the protagonist is reincarnated as a monster, they often have to fight for their lives in a world that may be hostile. For example, in 'So I'm a Spider, So What?', the spider has to constantly find food and avoid being killed. Another theme is growth. They usually start weak and gradually become stronger through various means like leveling up or evolving.
Another theme is world - building. Since the protagonist has god - like status, they often have a role in shaping or influencing the world they are in. They might introduce new systems, technologies or social orders. In some novels, the reincarnated god - like character may also face moral and ethical dilemmas regarding how to use their power.
One common theme is the struggle for power. The reincarnated protagonist usually starts from a weak position and has to fight their way up through cultivation. Another theme is revenge. They might be seeking revenge for something that happened in their past life. And of course, the exploration of the unknown in the cultivation world, like discovering new techniques and places.
One common theme is self - discovery. Since the character is reincarnated as a girl, they often have to figure out their new identity, which includes dealing with different social expectations compared to their previous life. For example, in 'So I'm a Spider, So What?', the spider girl has to learn about her new body's capabilities and limitations.
Common themes include identity crisis. The princess is no longer in her original life, so she has to figure out her new identity. Another is the theme of friendship. She forms new relationships with people in the new world. And often, there is a theme of overthrowing the status quo. The princess may use her new - found knowledge or powers to change the way things are in the kingdom or society she's in.
The exploration of a new world is another typical theme. When reincarnated as a monster, the character is in a different world or at least a different part of their original world. They get to discover new magic systems, cultures of other monsters, and hidden secrets. For instance, in many of these novels, the protagonist discovers unique magic spells or abilities that are specific to the monster species they have been reincarnated into.
Power acquisition and growth is also a theme. They start from scratch in their new lives as a girl and often have to gain power or skills to survive or achieve their goals. This can involve magic, combat skills, or social influence, and their journey in obtaining these things while being a girl can be full of interesting twists and turns.