Greninja might develop better control over its powers. In the fanfiction, it could start with some wild and uncontrolled water attacks, but with Delphox's help and the challenges they face, it hones its skills. Delphox, on the other hand, could develop a stronger sense of loyalty. Initially, it might be more focused on its own performance, but as it battles alongside Greninja, it realizes the importance of teamwork and loyalty to its partner.
One possible development for Greninja is that it could learn new techniques from Delphox. For instance, by observing how Delphox uses its psychic powers to manipulate fire, Greninja could find a way to use its water attacks in a more strategic and precise way. For Delphox, it could become more humble. When Greninja shows off some amazing feats with its speed and agility, Delphox might realize that there are always things it can learn from others and become less prideful.
Greninja could become more patient. Maybe at the start it's always eager to jump into battle, but through its experiences with Delphox, it learns to wait for the right moment. For Delphox, it might become more empathetic. Since Greninja has a different fighting style, Delphox could learn to understand and adapt to it.
Well, it could be a story about their adventures together. Maybe they team up to face a powerful enemy in a tournament. Greninja's speed and combat skills combined with Delphox's powerful fire - based attacks would make them a formidable team.
One possible plot development could be that Ash and Serena have to deal with the new responsibilities of having a baby. This might cause some tension between them as they figure out how to balance their own goals and the needs of the family. Lemon could be involved as a support system, or perhaps as a source of conflict if there are romantic feelings involved among the three. For example, if Lemon has feelings for Ash, Serena's pregnancy could make those feelings more complicated.
For Ash, he could develop better communication skills. In the Kalos League, he often gets so focused on battles that he forgets to share his thoughts and feelings with Serena. By learning to communicate better, he can strengthen their relationship. For Serena, she could develop more self - confidence in her own Pokémon training skills. Maybe she starts out doubting herself but as the Kalos League progresses, she becomes more assertive and makes her own mark in the Pokémon training world.
It's possible that Greninja's return is part of a prophecy in the Alola region. The local people might see Greninja as a sign of great change or a hero. So Ash and Greninja have to work together not just to battle but also to fulfill this prophecy, which could involve saving the Alola region's natural balance or something similar.
Ash could become more patient. Since Togepi is a relatively weak and cute Pokémon at first, Ash has to be patient in training it. This would be a change from his sometimes impulsive nature in battles.
A simple fanfiction could be that Greninja and Delphox are forced to be allies when their trainers are kidnapped by an evil organization. They have to put aside their differences and work together to rescue their trainers. Greninja uses its agility to scout the enemy base, while Delphox uses its fire attacks to create distractions.
One great idea for a fanfiction could be a story where Greninja and Delphox team up to face a powerful, mysterious new Pokemon that threatens their region. They have to combine their unique skills, Greninja's speed and water - based attacks with Delphox's fire - magic and psychic abilities.
Ash might develop a deeper sense of responsibility. In a fanfiction, if he is in charge of protecting Manaphy, he would have to think more about the consequences of his actions. He could also learn to be more diplomatic, especially when dealing with May's different approach to Pokémon. May could grow in her ability to adapt. For example, if they end up in a situation where there are no contests but only battles, she could learn from Ash how to handle it. Manaphy could develop its psychic abilities further. Maybe it discovers new powers within itself while being with Ash and May, and they help it to control and use these powers for good.
As a Mew fan, Ash might start to study different aspects of Pokémon lore more deeply. This could make him wiser in the ways of Pokémon. He could also develop a more empathetic side towards Mew - like Pokémon, seeing them as not just creatures to battle with but also as beings with their own unique qualities and stories.
Ash could become more understanding of different training methods through Daisy. Maybe he learns from her how to better communicate with his Pokemon on an emotional level.