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.
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.
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 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.
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.
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.
Definitely their wedding. It was a huge step for both of them, especially considering the challenges they had faced. It was a celebration of their love in the face of all the odds. Also, the time when Alec stood up to his family and the Shadowhunter society for Magnus was really powerful.
One great Alec Lightwood fanfic is 'The Light in Alec'. It beautifully explores Alec's character growth. Another good one is 'Alec's Journey' which focuses on his relationships within the Shadowhunter world. And 'Alec's New Path' is also a popular fanfic that shows Alec in different and exciting scenarios.
One idea for an Alec Lightwood birthday fanfic could be to focus on a surprise party his friends throw for him. They could transform the Institute into a magical wonderland with enchanted lights and floating decorations. Magnus could use his magic to create a unique gift, like a constellation of Alec's favorite stars that can be seen from inside the Institute. His sister Izzy might make his favorite meal, and Jace could present him with a new weapon he's been longing for.
One common way could be through using his magic in a subtle way. For example, he might use magic to create situations where Alec has no choice but to follow his lead. Maybe he makes it seem like the only option for Alec to protect his friends or family is to do what Magnus wants.