For Ash, his character can be developed by showing his growth in understanding psychic - type Pokémon. Since Sabrina is a psychic trainer, his interactions with her can make him more aware of different strategies. For Sabrina, she can develop by becoming more empathetic. Ash's bold and sometimes reckless nature can teach her to be more open - minded. For example, Ash might introduce her to some new ways of training that she hadn't considered before.
One interesting plot idea could be Ash getting lost in Sabrina's psychic maze and having to rely on his Pokémon's instincts more than his own. Another could be Sabrina having a premonition about Ash's future battles and deciding to train him in psychic - type moves. Or perhaps there's a Pokémon that is causing chaos in the town and Sabrina and Ash have to team up to stop it, which also helps Sabrina to open up more and become less aloof.
One idea could be Ash and Sabrina teaming up to face a new and powerful Pokemon that threatens their region. They have to combine their unique skills and knowledge of Pokemon to defeat it.
Ash's Pokemon would likely have mixed reactions. Some like Bulbasaur, being the gentle and observant one, would be quick to accept Sabrina. It might even enjoy the new psychic powers that Sabrina could bring into their group. However, Squirtle might be a bit mischievous at first, trying to test Sabrina's patience. For Sabrina's Pokemon, Alakazam, being highly intelligent, would sense Ash's good intentions towards Sabrina and would support the relationship. Her other Pokemon would follow Alakazam's lead.
For Ash, he could be developed by facing a situation where his usual courage and determination are not enough. Sabrina could be the one to guide him through it, making him learn new strategies and become more patient. This would add a new layer to his character.
One possible storyline could be that Ash, after many adventures, returns to Saffron City where Sabrina is. Their relationship could start from Ash challenging her gym again but this time they see each other in a different light. Maybe Pikachu plays a role in bringing them closer, like it gets lost in Sabrina's psychic maze and Ash has to work with Sabrina to find it. This shared experience could be the start of their romantic journey.
One common theme is the power of friendship evolving into love. They start as friends or acquaintances in the Pokemon world and as they interact more, feelings of love develop. Another theme could be overcoming differences. Ash is more of an energetic and sometimes reckless trainer while Sabrina is calm and powerful. They have to overcome these differences to be together.
Yes, there are. For example, some fanfictions might depict their adventures together where their relationship gradually develops. They could be on a journey to discover new Pokemon and during that time, Ash starts to see Sabrina in a different light, and Sabrina also becomes more open to Ash.
It really depends on the story. In some fanfictions, Sabrina might be more dominant. Given her psychic powers, she can control situations easily. For example, she could use her powers to influence the outcome of battles or make quick decisions in dangerous situations.
One interesting story could be about Ash and Sabrina teaming up to face a powerful, new, and mysterious Pokemon. They might use their unique skills and bond to overcome the challenges.