It would likely be a mix of respect and conflict. Bruce would respect his father's experience as Batman, but they might have different ideas about how to protect Gotham. For example, Thomas might be more willing to take extreme measures, while Bruce is more about protecting the city's soul as well as its people.
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.
The encounter could be filled with tension. Bruce might be angry at first, blaming Thomas for leaving him so early in life. But as they talk, they start to understand each other better. Thomas could share his own hopes and dreams for Gotham that he never got to fulfill, and Bruce could see a new side of his father. The description could include details of their body language, like Bruce clenching his fists at first and then gradually relaxing as they talk.
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.
Their relationship could develop through a series of adventures. For example, they might have to stop a powerful villain together. In the process, Damian's impulsive nature could clash with Thomas' more methodical approach. However, over time, they realize that they can complement each other. Thomas could help Damian think more strategically, while Damian could inspire Thomas to be more bold and take risks in their fight against evil.
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.
One possible plot could be that Thomas Wayne somehow time - travels to the present where Bruce Wayne is Batman. Thomas could initially be shocked at how Bruce has turned out, but then come to understand his motives. He might offer some old - fashioned wisdom that helps Bruce in his fight against crime.
The fanfiction often delves into the emotional connection between the two. Alfred is the one stable presence in Bruce's life. He is there to pick up the pieces when Bruce fails or has a setback. And Bruce, in turn, respects Alfred's wisdom and advice. In some fanfictions, Bruce might confide in Alfred things he wouldn't tell anyone else, like his doubts about whether he can really make a difference in Gotham. This shows the deep trust between them.
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.
Often, it starts with curiosity. Damian may be curious about his grandfather he never met. Thomas, on the other hand, is intrigued by this young Wayne. As they interact more, respect grows. Thomas sees Damian's potential and Damian looks up to Thomas as a part of his family's great history.
At first, it might be a bit rocky. Harry is used to the wizarding world and its ways, while Bruce has his own set of rules in Gotham. But over time, they'd start to understand each other. Harry would learn about justice in a different way from Bruce, and Bruce would come to respect Harry's magical abilities.
Bruce's life as Batman would change drastically. He would now have someone depending on him not just as Bruce Wayne but also as Batman. He might have to cut down on his nightly patrols or find more secure ways to keep the baby safe while he's out fighting crime. He would also be more motivated to make Gotham a safer place for his baby to grow up in, so he could focus on taking down the major threats to the city more quickly.