Well, you could have a plot where Magnus is showing Alec a part of his magical past for the first time. Maybe he takes Alec to an ancient magical realm that was important in his life. There, Alec discovers new things about Magnus and also about the wider magical world. Or perhaps it's the first time they are celebrating a special Downworlder holiday together. They try to incorporate both of their cultures into the celebration and it becomes a really sweet and memorable time for them. Also, it could be about their first time living together and all the little adjustments and cute moments that come with that.
A great plot idea is that it's their first time collaborating on a big mission to save the Shadow World. They have to rely on each other's strengths - Magnus's magic and Alec's leadership skills. Along the way, they share some really tender moments that strengthen their bond. Another could be that it's the first time Alec is introducing Magnus to his family in a more formal way. There are some tense moments as Alec's family is still getting used to Magnus being a warlock, but in the end, they accept him. And then there's the idea of their first time traveling to a different country in the magical world, experiencing new cultures and having adventures specific to that place.
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.