Often, themes of acceptance are present. In 'undertale gay fanfic', the characters may have to deal with the reactions of others in the Undertale universe regarding their relationship. Another theme could be self - discovery, where a character realizes their feelings for another male character and how that changes their view of themselves.
One common theme is probably the exploration of relationships between male characters in the Undertale world. It could be about how they support each other and grow closer.
Love and acceptance are common themes. Just like in any relationship story, in gay Undertale stories, the characters often have to face the challenge of being accepted by their friends, family (if applicable in the Undertale context), and the community around them. They also deal with self - acceptance, realizing and coming to terms with their own sexual orientation within the framework of the Undertale world.
Another theme is the struggle between good and evil. The ocs can be faced with moral choices, whether to side with the more 'good' characters in Undertale or be tempted by the 'evil' forces. This adds depth to the story and makes it more interesting.
One common theme is the change in relationships. For example, how the character's partner or friends react to the pregnancy. Another theme is the anticipation and fear of the unknown. Since the Undertale world has its own unique elements, the pregnancy might bring new challenges and uncertainties. Also, the theme of protection often comes up, with the character and those around them being protective of the unborn.
One common theme is self - discovery. The monster OCs often start out not really knowing who they are or what they can do, and then through their adventures in the Underground, they learn more about themselves. For example, they might discover hidden powers or a new purpose in life.
One common theme is redemption. Many fanfics try to show Chara in a new light, as a character who can be redeemed despite her actions in the game. For example, they might explore her backstory to find reasons for her behavior and then show her growing and changing for the better.
Identity is also a big theme. The undertale characters are put in new situations in the supernatural crossovers, and they start to question who they really are. For instance, if Chara is exposed to a world of angels and demons in a crossover, they might wonder if they are truly a 'bad' character or if they can change and find a new purpose in this new and complex supernatural setting.
The exploration of morality is a theme that can be found. In Undertale, morality is a huge part of the gameplay with choices between mercy and killing. In BATIM, there are also moral dilemmas in a sense, like whether to trust certain characters or not. Fanfics might combine these moral aspects, having characters from both face new moral choices in the crossover world. Also, the theme of discovery is there, as characters discover the new rules and beings of the other's universe.
One common theme is redemption. Since Chara has some dark aspects in the game, many fanfics explore the idea of Chara being redeemed through their relationship with Sans. Another theme is friendship. Despite their differences, they can form an unlikely bond. And there's also the theme of mystery, as both characters have elements of mystery in the original game, and fanfics often expand on that.
The theme of transformation is also prevalent. It's not just about the physical change from male to female but also the emotional and mental transformation. In many of these fanfics, the character has to adapt to new ways of thinking, new social roles, and new ways of interacting with the world around them. This is often explored through their interactions with the unique and diverse characters in the Undertale setting.
One common theme is redemption. Since Asriel has a complex past in Undertale, many fanfics use the relationship with Sans to explore his path to redemption. Another is friendship. Despite their differences, they can form a unique bond.