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.
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 common theme is unrequited love. Maybe Ash has feelings for a character who doesn't feel the same way back. Another could be betrayal. For example, a friend or a loved one betrays Ash, which leads to his heartbreak. Also, the theme of loss can be present, like losing a Pokemon that was very close to him, which shatters his heart in the fanfiction.
Well, perhaps in the fanfiction, Ash is betrayed by a group of friends he trusted deeply. They might have used him to get access to some rare Pokémon or some important information, and then they turn their backs on him. This would be a huge heartbreak for Ash, who is always so loyal to his friends. Or it could be that he fails to achieve a major goal like winning a big Pokémon tournament due to some unfair circumstances, and he feels heartbroken as all his efforts seem to be in vain.
It could be that there is some misunderstanding between them. Say, Misty misinterprets something Ash does as him not caring about her anymore. Ash, being clueless about her misinterpretation, doesn't correct it in time. Then Misty distances herself from him, and Ash is left heartbroken when he finally realizes what has happened.
It could slow him down. Ash might be so distraught that he doesn't focus well on training his Pokemon. His usual enthusiasm for battles and catching new Pokemon might wane, as his mind is preoccupied with his broken heart.
Loss is a major theme. It could be the loss of a Pokémon, a friendship, or a dream. If Ash loses a Pokémon to an incurable illness or if his dream of becoming a Pokémon master seems out of reach, it would cause him to be heartbroken.
A theme that often pops up is the idea of growth. Heartbreak can be a catalyst for Natsu to grow as a character. He might become more empathetic or learn to be more cautious in relationships. Also, the theme of forgiveness can be present. He may have to forgive the person who caused his heartbreak to move on.
There might be a 'ash is heartbroken fanfiction' where Ash discovers that his long - time rival has surpassed him in a way he never expected. This makes him question his own journey and abilities, leading to heartbreak. For example, if Gary becomes the Pokémon master before him and Ash feels like all his efforts were in vain.
One popular 'natsu gets heartbroken fanfiction' could be 'A Broken Heart's Journey'. It delves into Natsu's emotional turmoil after a betrayal from a close friend. The story shows his struggle to come to terms with the pain and how he gradually tries to heal.