It could be his arch - enemy. For instance, the Joker might play a cruel trick on Nightwing that makes him lose someone he cares about deeply, thus causing his heartbreak.
Even another superhero might cause his heartbreak. Maybe there's a power struggle within the superhero community. A more powerful and ambitious hero could undermine Nightwing, steal his credit for a big victory, and spread lies about him. This would make Nightwing feel betrayed and heartbroken.
A possible plot could be Nightwing getting betrayed by a close friend or ally. Maybe they were framed for something they didn't do, and this led to their heartbreak. For example, if a fellow hero believed false rumors about Nightwing and turned against him, it would shatter his trust and break his heart.
To effectively portray Nightwing's heartbreak, the author can make use of flashbacks. Show the happy times he had before the heartbreak, which will contrast sharply with his current state. The author can also have him make self - deprecating remarks. For instance, 'I'm such a fool for believing in love.' Additionally, his interactions with the environment can be a sign of his heartbreak. He might avoid going to places that remind him of the person or event that caused his pain.
It could be Annabeth. Maybe they had a huge fight over something important, like the future of their relationship or a difference in their values regarding their demigod duties. And this fight led to Annabeth leaving Percy, which broke his heart.
Often, in Nightwing heartbroken fanfics, a common theme is disappointment. For example, Nightwing might have high hopes for a mission or a new partnership, but it all goes wrong. It could also be about self - doubt. After a series of bad events, he starts to doubt his own abilities and worth, which adds to his heartbroken state. The theme of isolation can also be present. He might feel so heartbroken that he isolates himself from his friends and family.
Another possibility is that the Pokemon themselves could be the cause, in an unexpected way. For example, if one of Satoshi's Pokemon gets seriously ill and needs to be left in a far - away place for treatment. Satoshi decides to stay with the Pokemon, but Haruka has to continue her journey. This difficult decision could lead to heartbreak for both of them.
One possible fanfic could involve a battle against a powerful villain. Nightwing might get injured while trying to protect Gotham. Batman then has to step in not only to defeat the villain but also to take care of Nightwing. He could use his advanced medical knowledge and resources in the Batcave to nurse Nightwing back to health.
Gary could also be the one to cause Ash's heartbreak. Maybe Gary outshines Ash in a big Pokemon battle and then mocks him for it. This could make Ash feel like a failure and break his heart. Gary has always been a bit of a rival to Ash, so this kind of situation is plausible in fanfiction.
Another character could be Kakashi. Maybe he has to make a tough decision that goes against Fem Naruto's wishes. For example, not allowing her to go on a dangerous mission she really wants to. This could make Fem Naruto feel betrayed and heartbroken, as Kakashi is a respected figure in her life.
A new, powerful villain could be the cause. This villain might target Ash's Pokémon and manage to steal them all away. Ash would be heartbroken not only because he lost his Pokémon but also because he feels he has failed to protect them.
One idea could be a story where Beastboy and Nightwing have to team up for a mission in a new, mysterious part of the city. They start off not getting along well due to their different personalities and fighting styles. Beastboy is more carefree and impulsive while Nightwing is methodical. But as the mission progresses, they learn to appreciate each other's strengths and form a strong bond by the end.