Well, there's this fanfic called 'Love in the Shadows'. It's really interesting because it shows how Bruce and Rachel's relationship is affected by the whole Batman thing. Bruce is constantly torn between his duty as Batman and his love for Rachel. And then there's 'Rachel's Discovery', where Rachel starts to figure out Bruce's double life and how that impacts their relationship in a very complex way. There's also a fanfic that focuses on their childhood memories together, 'Memories of Bruce and Rachel', which is a really sweet take on their relationship.
One common theme is the struggle between love and duty. Bruce has his duties as Batman, and this often clashes with his relationship with Rachel. Another theme is the idea of redemption. Rachel can sometimes be seen as a symbol of normalcy and goodness that Bruce is drawn to, and his relationship with her is a way for him to find redemption from his dark and violent life as Batman.
Often, she is depicted as being just as intelligent as the male Bruce Wayne. For example, she has a sharp mind for detective work and strategy. She might be able to quickly analyze crime scenes and figure out the motives of the villains.
He might develop a greater sense of humility. Without the Batman persona to fall back on, he has to face his own limitations as a person. For example, he may struggle to get people to listen to his ideas for change when he doesn't have the intimidation factor of Batman.
One great fanfic is 'The Father - Son Bond'. It beautifully explores the complex relationship between Bruce and Damian. It shows how Bruce tries to be a good father while also dealing with his role as Batman, and Damian's struggle to fit into this new world with his own strong - willed nature.
The plot is also important. It could involve them facing a new villain or a threat to Gotham that tests their relationship. Maybe a situation where Damian's actions put him at odds with Bruce's code of ethics, and they have to work through it. This kind of plot tension can make the fanfic really engaging.
At first, it could be a bit rocky. Bruce is used to his own way of doing things and Buffy has her own independent streak. But they would gradually learn to respect each other. Bruce might be impressed by Buffy's combat skills and her ability to handle the supernatural. Buffy, on the other hand, could learn a lot from Bruce's strategic thinking.
Believable young Bruce Wayne is about showing his evolution. Start with his basic characteristics. He's smart, already showing signs of being a prodigy. But then add in the emotional aspects. He's angry about what happened to his parents but channels that anger into something positive. He might start small, like helping a neighbor who has been robbed. His relationships with other kids his age are also important. He might not fit in completely because of his different mindset. And his growing obsession with bats could be a unique quirk that you play up, as it ties into his future identity as Batman.