It could be a plot twist if they get back together only to find out that there is a curse on their relationship. Every time they are happy together, something bad happens to someone they care about. They then have to decide if they are willing to sacrifice their own happiness to protect their loved ones or find a way to break the curse, which could lead to some really intense and emotional scenes in the fanfiction.
Perhaps a new character could be introduced who tries to come between them at first but actually ends up pushing them closer. This character might start dating one of them, making the other jealous. But then, they both realize that they are truly meant for each other through this complicated love triangle situation. As they work through their feelings of jealousy and confusion, they find their way back to each other.
A theme could be the reactions of their friends and family in the story to the pregnancy. This could add drama and different perspectives to the fanfic.
One possible plot twist could be that they get back together due to a common enemy. Maybe a new threat emerges in their world and they realize that they work better as a team when they are together. So they put aside their past differences and start to re - kindle their relationship.
A really cool fanfiction could be set in a modern day reboot of their story. Tate and Violet are reincarnated in a different time, but still have a connection. They start to remember their past lives slowly as they are drawn to each other in this new world. They have to deal with modern problems while also coming to terms with their haunted pasts.
One common theme could be the reactions of the other characters to Violet Tate's pregnancy. For example, her friends and family might have different emotions, from shock to joy.
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 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.
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.