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 scenario could be that Satoshi and Haruka have a misunderstanding. Maybe Satoshi made a promise to Haruka that he couldn't keep, like promising to always travel with her but then having to leave for an important mission. This could lead to heartbreak as Haruka feels betrayed and Satoshi feels guilty.
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.
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.
Satoshi is often a very liked character as he is the main protagonist in the Pokemon anime and his adventures are well - known. Fans like to see him in different scenarios 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.
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.
One interesting plot could be Satoshi and Haruka teaming up to discover a new and rare Pokemon species. They might face challenges like difficult terrains and other trainers trying to get to the Pokemon first.
It could be a new villain. Maybe a villain who targets Naruto's loved ones and Naruto is unable to save them. This failure would cause him great heartbreak. For instance, if the villain kidnapped Hinata and Naruto couldn't rescue her in time.
It could be Giovanni. If Giovanni gives an order that Meowth finds completely unacceptable, like harming innocent Pokemon for no good reason. Meowth, having a soft heart deep down, might not be able to go along with it and thus betray Team Rocket.