One possible main theme could be love. Their relationship might be central to the story, showing how two different characters can come together and support each other.
Love is a very common theme. Their relationship, which is between a Shadowhunter and a warlock, is full of challenges and the fanfics often explore how they overcome these through love. Another theme is acceptance. Alec has to accept Magnus's different lifestyle as a warlock, and Magnus has to accept Alec's duties as a Shadowhunter.
Another theme is the struggle against prejudice. Since they are from different 'worlds' in the story, they face a lot of judgment from others, and fanfictions often deal with how they overcome this. Also, the theme of self - discovery is quite common. Both characters grow and learn more about themselves through their relationship with each other.
Love is a very common theme. Their love story, how they overcame differences to be together. Another is the theme of acceptance, like Alec accepting Magnus' warlock nature and Magnus accepting Alec's Shadowhunter duties. And also the theme of family, as they both have different family backgrounds and how they merge their families in a way.
A frequent theme is also the power dynamics. Magnus has his powerful magic, and Alec has his position in the shadowhunter world. How they balance this in their relationship is often explored. Also, the theme of family acceptance is common. Alec's family and their reactions to his relationship with Magnus are often part of the stories.
One common theme is the exploration of new relationships. The addition of an OC often creates a new dynamic between Alec and Magnus. For example, the OC could be a mediator in their relationship, helping them understand each other better.
Another theme is self - discovery. In human fanfictions, they might be on a path to find out who they are without the trappings of their original magical or shadowhunter identities. They learn about their strengths and weaknesses as normal humans.
Love and acceptance are big themes. In many fanfics, Alec has to come to terms with his feelings for Magnus, a warlock, which goes against what he has been taught as a Shadowhunter. Also, there is the theme of sacrifice. Sometimes one of them has to sacrifice something for the other's safety or well - being, which really shows the depth of their love.
A major theme in magnus x alec fanfiction is the exploration of power dynamics. Magnus is a powerful warlock and Alec is a shadowhunter. How they balance their different levels of power and influence in their relationship is often explored. Also, the theme of acceptance is quite prevalent. They come from different worlds, and their journey towards accepting each other's differences is a key part of many stories. For example, in some fanfictions, Alec has to come to terms with Magnus's long life and his different magical practices.
One common theme is love and relationship development. It shows how Magnus and Alec's relationship grows and overcomes various obstacles. Another theme could be adventure, where they go on quests together while being in a relationship.
In the 'alec and magnus book story', if it's in a fantasy setting, they could be part of different species or groups. Maybe Alec is from a more traditional or conservative group, and Magnus is from a more flamboyant and powerful magical group. Their coming together could be a source of both conflict and unity. The story might explore themes like acceptance, both within their own communities and between each other. There could be exciting plot twists involving magic spells, ancient prophecies that they are a part of, or even political intrigue within the magical world that they have to navigate.