He might isolate himself at first. Natsu could withdraw from his friends and normal activities, needing time alone to process his pain. This is a common way for people, or in this case, characters, to deal with heartbreak.
He might isolate himself. Naruto has always been a social person, but the heartbreak could be so intense that he just wants to be alone. He might go to his favorite training spot and just train by himself for days, trying to forget the pain through physical exertion.
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.
Definitely Lucy. Their bond is so strong that if there were any cracks in it, like if Lucy started distancing herself from Natsu without a clear reason, it would be devastating for him. Gray is also a candidate. They've been through so much together, and if Gray were to say or do something really hurtful, like blaming Natsu for a big mistake that wasn't his fault, it could lead to heartbreak. Juvia might be involved too. If she tried to keep Lucy away from Natsu for her own love for Gray, it could cause problems for Natsu and lead to his heartbreak.
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.
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.
One possible reason could be unrequited love. Maybe Natsu has feelings for someone who doesn't feel the same way about him. This could lead to his heartbreak as he realizes that his love is not reciprocated.
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.
Well, it could be due to the failure to protect the village. Naruto has always been about protecting Konoha. If something really bad happened, like a major attack that he couldn't prevent and many lives were lost, he might feel so much guilt and heartbreak that he decides to leave. He would feel like he failed his purpose.
Naruto might throw himself into his ninja training. He's always been dedicated to getting stronger, and heartbreak could fuel that drive even more. For example, he could spend countless hours perfecting new jutsu or sparring with his friends. Another way he could cope is by relying on his friends. Sakura and Kakashi, for instance, could be there to support him, listen to him vent, and offer advice. He might also find solace in eating ramen at Ichiraku. The familiar taste and the warm atmosphere of the ramen shop could give him some comfort.
A great natsu heartbroken fanfiction is 'When Natsu's Heart Broke'. It explores a situation where Natsu is deceived by someone he trusted, leading to his heartbreak. The story is full of his internal struggles and how he tries to move forward.