A story where they have to deal with a threat to the Shadowhunter world that specifically targets their relationship. They have to fight not only the external enemy but also the doubts and insecurities within their relationship. It could involve a lot of emotional drama and also some great action scenes as they protect each other.
One plot idea could be the three of them going on a dangerous magical quest together. They have to face various challenges and in the process, their relationships deepen. Maybe Magnus uses his powerful magic, Alec shows his leadership and combat skills, and Jace his quick thinking.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions in various fandoms, and you're likely to find some great magnus x alec x jace ones there. Just use the search function on the site.
One plot idea could be that the reader is a new Shadowhunter in the Institute and Jace has to train them. This would lead to a lot of interaction, from sparring sessions where they start to develop feelings for each other through physical closeness and mutual respect for each other's skills, to Jace protecting the reader during a dangerous mission against demons.
One idea could be that they adopt a young warlock who is having trouble controlling their powers. Alec and Magnus have to teach the child how to manage their magic while also protecting them from those who might want to use the child's power for bad. Another plot could be about adopting a mundane child and introducing them to the Shadow World. It would be full of discovery and the child learning about their new family's unique abilities.
One plot idea could be that Magnus and Alec go on a magical adventure together for the first time. They encounter a mysterious magical creature that is causing chaos in the Shadow World. As they work together to solve the problem, their relationship deepens. Another idea is that it's their first time attending a Downworlder - Shadowhunter event together. There they face some prejudice from both sides but their love for each other helps them overcome it. A third plot could be that they have their first real argument and then make up in a very romantic way, exploring more about their different personalities and how they can complement each other.
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.
A really cool plot could be that there's a prophecy that involves both of them. The prophecy says that only by combining their unique abilities - Percy's control over water and Magnus's connection to the Norse gods - can they save both the Greek and Norse mythologies from being wiped out by a new, unknown force. They have to go on a journey, meet various characters from both mythologies, and overcome many challenges along the way.
One plot idea could be that Rodimus and Ultra Magnus have to team up to face a new, powerful enemy that neither could defeat alone. Their different leadership styles - Rodimus' more impulsive and Ultra Magnus' by - the - book - could cause some initial friction but they learn to work together effectively.
One idea could be that Alec is really nervous about meeting Magnus' dad. Maybe he tries to learn all about Magnus' family history to make a good impression. He could bring a special gift that has significance in Magnus' family's culture.
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.
One great 'magnus x alec fanfiction' is 'The Bond of Magic and Love'. It beautifully portrays their relationship development with well - written dialogues and vivid descriptions of their magical world. Another is 'Magnus and Alec: A Journey Together' which focuses on their adventures side by side, showing how they support each other through tough times.