In a fanfiction, perhaps there has been some external threat that forces them to work together again. As they face this danger side by side, they remember why they were so in love in the first place. Their shared memories and the adrenaline of the situation bring them closer, and they gradually start to get back together.
One possible plot could be that they meet by chance after a long time apart. Maybe at a Downworlder event where Magnus is using his magic to create some amazing display. Alec, who is there on some Shadowhunter business, can't take his eyes off Magnus. They exchange a few words, and the old feelings start to resurface.
Trust could be a big issue. Since they've broken up before, there might be lingering doubts. Alec might worry that Magnus will leave again, and Magnus might be afraid that Alec will let the rules of the Shadowhunters come between them. They need to work through these insecurities if they want to get back together.
Well, it varies in different fanfics. Sometimes it's a case of saving each other's lives, sparking romantic feelings. Other times, it could be a series of heartfelt conversations that make them realize their love for one another.
Well, perhaps the fanfiction could focus on how Magnus and Alec adjust to having a baby in their lives. Alec, being a shadowhunter, might have some traditional ideas about parenting which could clash with Magnus's more free - spirited and magical approach. And they could grow and learn from each other throughout the process.
Well, it could be a moment of intense emotion. Maybe they've been through a lot together, like fighting some great evil side by side. And in a moment of relief after a big victory, their eyes meet and they just can't hold back any longer. The kiss could be soft at first, full of unspoken feelings that have been building up for a long time.
It could be that the accident makes Alec temporarily lose some of his abilities. Magnus, feeling responsible, devotes all his time to finding a way to restore Alec's abilities. Along the way, they face many challenges and also discover new aspects of their relationship as Magnus is forced to be more vulnerable about his fears of losing Alec.
One possible plot could be that after a long time apart, they accidentally meet at a place that holds significance for them, like the old Total Drama Island. Their initial encounter might be filled with awkwardness and old grudges resurfacing. But as they start talking, they realize they still have feelings for each other. Maybe Duncan has changed for the better during their time apart, and Courtney sees this new side of him that makes her fall in love all over again.
One possible plot could be that after a long time apart, Scott and Allison accidentally meet at a place that holds a lot of memories for them, like the high school where they first met. This encounter stirs up old feelings, and they start to talk about the past. Slowly, they realize that they still love each other deeply and begin to rekindle their relationship.
One possible plot could be that they meet by chance at a music event. Maybe Jesse is performing and Rachel is in the audience. After the show, they start talking and realize they still have feelings for each other. They then slowly start rebuilding their relationship by going on dates and reminiscing about their old times together.
One possible plot could be that they realize they still have feelings for each other during a chance encounter. Maybe at a mutual friend's party. They start talking, reminiscing about the good times, and slowly the old spark reignites.
The plot could be that their children (in a sense, the characters they care about) play a role in bringing them back together. Maybe the characters notice how much Asgore and Toriel still have feelings for each other. They set up situations where Asgore and Toriel are forced to interact, like a dinner or a shared task. Through these interactions, old feelings resurface, and they get back together.