Love and acceptance are very common. Given that they are from different worlds (Shadowhunter and warlock), their relationship often involves them learning to accept each other's differences. Another theme is family. Alec has his Shadowhunter family and Magnus has his own kind of 'found family' among the Downworlders. Fanfictions often explore how their relationship affects and is affected by their respective families.
There are also Tumblr blogs dedicated to sharing Alec Lightwood x Magnus Bane fanfiction. You can search for relevant tags and find some really unique and creative works that might not be as mainstream as those on AO3 or Fanfiction.net. These can offer fresh takes on their relationship, such as exploring how they handle cultural differences between the warlock and Shadowhunter worlds.
One of the most popular themes is the exploration of their cultural differences. Magnus being a warlock with a long and rich history and Alec being a Shadowhunter from a more traditional background. Another theme is their struggle against prejudice. They face discrimination from the Clave and others due to their relationship. Also, the theme of sacrifice is common, like how they would sacrifice for each other's safety and happiness.
One great Magnus Bane and Alec Lightwood fanfiction is 'The Bond Beyond Magic'. It beautifully explores their relationship, delving into how their different personalities complement each other. Another is 'Alec's Discovery' which focuses on Alec's journey of self - discovery within the relationship. And 'Magnus' Heart' shows Magnus' perspective and his deep love for Alec in a very touching way.
There could be some challenges too. Magnus is used to a certain dynamic with Alec, and now he has to adjust to Alec being more vulnerable. This might cause some minor frictions at first, but ultimately it could strengthen their bond as they learn to navigate this new phase together.
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.
Alec and Magnus Bane's love story is really beautiful. They are from the 'Shadowhunters' world. Their relationship started with a bit of hesitation from Alec due to his Shadowhunter upbringing. But Magnus's charm and his own growing feelings led to a deep and passionate love. They faced many challenges together, like dealing with the prejudices of the Shadowhunter society against warlocks. Their love overcame all these difficulties and became a symbol of love that transcends boundaries in that fictional universe.
Well, in Magnus x Alec lemon fanfiction, a frequent theme is the discovery of each other's desires. Since they come from different backgrounds, Magnus being a warlock and Alec a Shadowhunter, there's a lot of room for exploring how they understand and fulfill each other's needs. Additionally, themes of acceptance, both within themselves and from their respective communities, are quite common. The 'lemon' aspect can sometimes be used to show the deepening of their bond and the breaking of barriers between them.
Once upon a time, Alec Lightwood, a shadowhunter, met a girl named Lily at a demon - hunting mission. Lily was a mundane with an uncanny ability to sense the presence of demons. Alec was initially hesitant, but as they spent more time together fighting demons side by side, he couldn't help but fall for her. Their love grew against all odds, as they had to hide their relationship from the Clave which didn't look favorably upon shadowhunter - mundane relationships.
They could also have a moment of friendship while exploring new places. For instance, they visit a hidden magical realm that neither of them has been to before. They share their excitement and fears during this adventure, relying on each other for support and protection, which deepens their friendship.