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.
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.
One interesting scenario could be Thomas Wayne seeing Damian's combat skills and being both proud and a bit shocked. After all, Thomas may not have expected such a young and fierce fighter in the family. Another could be them having a deep conversation about family values, with Damian learning from Thomas about the Wayne legacy from its very origin.
Thomas Wayne's relationship with the Bat Family would develop in different ways. With Alfred, they would likely share a mutual respect as they both care deeply about the Wayne family. Thomas would probably be impressed by Alfred's loyalty and service. Regarding the female members of the Bat Family, such as Barbara Gordon, he could offer his support in their fight for justice, seeing them as an important part of Gotham's protection. His presence could also bring a new sense of family unity to the Bat Family, making them more cohesive in their crime - fighting efforts.
In certain comic arcs, Thomas Wayne is shown to be the father of the Joker. However, comic universes can vary greatly, and this isn't a universal truth for every Joker-related story.