One possible storyline could be exploring the immediate aftermath of Bruce's death. How does Terry McGinnis handle the loss? Another could be flashbacks to Bruce's final days, showing his last words of wisdom to Terry.
Bruce Wayne's death in Batman Beyond was a natural consequence of his life's journey. He had been Batman for a very long time, facing all kinds of challenges. His body had been through numerous injuries and his mind had been burdened with the weight of protecting Gotham. Over time, these factors combined with the natural aging process. His heart, which had pumped blood through his battle - worn body for so many years, finally stopped, and he passed away, but his spirit lived on in the new Batman, Terry McGinnis.
One reason could be that Bruce Wayne never had the traumatic event that led him to become Batman in the original story. Maybe his parents' death was prevented somehow, so he never felt the need for vengeance and justice in that extreme way.
Well, in such a fanfic, it might show Bruce Wayne as a completely different entity from Batman. Maybe Bruce is just a normal rich guy who has no idea about the vigilante activities of Batman. He could be living his own life, focused on business and social events, while Batman is someone else entirely, perhaps an unknown figure with no connection to the Wayne family.
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.
Bruce Wayne in Batman Begins fanfiction is often a complex character. He is a man on a journey of self - discovery and vengeance. In fanfiction, we might see different aspects of his training that were not fully explored in the movie. For example, his time with Ra's al Ghul could be delved into more deeply, showing his internal struggle between the teachings of the League of Shadows and his own moral code.
The dynamic could be complex. Thomas might be both proud and worried seeing Bruce as Batman. Proud that his son is carrying on the legacy, but worried about the dangers. He could be overprotective at first.
One possibility is that Thomas Wayne Batman comes from an alternate timeline. He meets Bruce Wayne in the current timeline and shows him a different way of approaching crime - fighting. Maybe he has more brutal methods, which causes Bruce to question his own more restrained approach. They could then learn from each other and create a new, more effective Batman strategy for Bruce's Gotham.
The author might introduce a new character to take on the role. Maybe a completely unknown vigilante steps up. This new character could have different motives and methods compared to Batman. For example, they could be more focused on preventing crime through community outreach rather than the intimidation tactics Batman uses.