The story could end well if the Titans manage to defeat the cause of his injury. For example, if it was a villain, they could capture or even rehabilitate the villain. And Beast Boy, through his experience, could grow closer to the other Titans and they could have a big celebration once he's fully healed.
One possible plot development could be that the other Titans rush to find a way to heal him. Maybe they have to seek out a rare herb or a powerful magic user. Another could be that Beast Boy's injury causes him to have strange side - effects, like losing some of his shape - shifting abilities temporarily. And it might also lead to some self - discovery for Beast Boy as he copes with his pain and weakness.
To write a Beast Boy hurt comfort fanfiction, start with a strong setup for the hurt. For instance, Beast Boy could be betrayed by someone he thought was a friend. This would be a great emotional wound. Next, think about how you want the comfort to be delivered. Maybe Beast Boy runs away and hides in the forest. Raven, being able to sense his distress, follows him. She doesn't say much at first but just sits beside him. As time passes, she shares her own experiences of feeling alone and how she overcame it. This slowly helps Beast Boy open up and start to heal. Include lots of description of their emotions and the setting to make it more engaging.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). There are likely to be many different stories there that involve Beast Boy getting hurt in various ways. Just use the search function and type in relevant keywords like 'Beast Boy hurt'.
To write a great fanfic about Jason being badly hurt, you need to research. Look at how other fanfics handle injuries. For the story, start with a strong opening that shows the moment of injury vividly. Then, develop the plot around his recovery. Include his friends' reactions, like Annabeth's strategic thinking on how to help or Percy's concern. And don't forget to add some internal monologue for Jason to show his pain and determination.
There's also the possibility that it's not a dark force at all. It could be a well - meaning but misguided wizard who accidentally causes Harry to be badly hurt. For instance, during a training session that goes wrong or while trying to perform a difficult spell to protect Harry but ends up harming him instead.
One example could be a story where Beast Boy gets injured during a battle with a powerful new villain in the city. He might struggle with his powers not working properly due to the injury, which adds a lot of drama to the fanfiction.
It could be an enemy. For example, a rival in a competition or a villain who has a grudge against Leo.
In the fanfiction, it might be that a mysterious figure is responsible. This person could have a vendetta against Amy's family or the stables she works at. They could set up a trap, like loosening a fence rail that Amy's horse jumps over, causing the horse to stumble and Amy to be thrown and badly injured.
One possible reason for Natsu to return badly hurt in a fanfiction could be a very tough battle. Maybe he faced an extremely powerful enemy that was far beyond his initial expectations. For example, if he encountered a new type of dark mage with some unknown and dangerous magic. This could lead to him getting seriously injured and having to struggle to return.
One could be 'Jason's Painful Ordeal' where he gets seriously injured in a battle against a powerful new foe. His friends are left to deal with the aftermath, and it shows their grief and determination to save him.