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.
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.
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.
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.
Naruto may also try to find solutions. Despite the heartbreak, his optimistic nature might still shine through a bit. He could start to search for ways to mend the relationship with his parents, whether it's through magic in the Naruto world or by trying to prove his worth to them.
In this kind of fanfiction, Naruto being heartbroken by Rias could be due to a variety of reasons. Maybe Rias betrays Naruto in some way, like choosing someone else over him in a crucial situation. Or perhaps there are misunderstandings between them that lead to Naruto's heartbreak. It could also be that Rias has to follow some rules in her world that go against Naruto's expectations, causing their relationship to shatter.
One aspect could be that the fanfiction shows Naruto's heartbreak in a very visual and emotional way. For example, he might see his parents in a dream - like state and then they disappear, leaving him feeling more alone than ever. It's a way to dig deeper into his character's vulnerability, which was somewhat masked in the original series by his determination.
One possible reason could be the loss of a dear friend. Naruto values his friends deeply, and if he were to lose one, it would break his heart. For example, if Sasuke were to die or turn against him completely in a way that Naruto couldn't fix, it would be a huge blow.
Let's say he gets a weapon that can transform. For example, it can change from a small dagger to a large spear. This would make his combat style more versatile. He could start a fight with the dagger form for quick, sneaky attacks, and then transform it into a spear when he needs more power and reach. This new weapon would force him to learn new fighting techniques and adapt his style to make the best use of its abilities.
One possible reason could be unrequited love. Maybe Naruto has deep feelings for Anko, but she doesn't feel the same way. She might be focused on other things like her ninja duties or she could have feelings for someone else. Another reason could be that Anko betrays Naruto's trust in some way. For example, she might break a promise she made to him that was really important. It could also be that Anko does something that goes against Naruto's ideals, which makes him feel heartbroken as he had a different image of her in his mind.