It could also be due to competition. Sabrina might see Ash as a threat to her status as a powerful trainer. If Ash starts to gain more recognition and success, Sabrina, being prideful, could feel betrayed as if Ash is trying to overshadow her and thus take actions against him.
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.
One possible analysis could be that in such a fanfiction, the idea of Ash being betrayed might be a way to create drama. It could involve his friends or companions in the harem turning against him for various reasons like jealousy or misinformation.
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.
One common plotline could be that Sabrina, perhaps feeling overlooked or underestimated by Ash, decides to betray him in a Pokémon battle. She might use underhanded tactics to win, which causes a rift between them.