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.
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 Joker has killed Thomas Wayne in specific comic iterations. This plot twist adds depth and complexity to the characters' relationships and the overall narrative.