One common plotline might be Harry's discovery of his true paternity. He could struggle with the idea of being related to Grindelwald, a dark wizard. Another could be the reaction of his friends, like Hermione and Ron, who might be shocked and confused.
In these fanfictions, a main plotline could be that Grindelwald had some grand plan in which Harry was an important part, even though he didn't know Harry was his son. So as Harry uncovers his true identity, he has to figure out what Grindelwald's intentions were all along. There might be a race against time as Harry tries to prevent whatever dark plan Grindelwald had in store, all while dealing with his own internal conflict of being related to such a notorious dark wizard.
Often, there's a plotline where the wizarding world has to deal with this new revelation. Some might see Harry as a threat because of his relation to Grindelwald. There could also be a search for more information about how this came to be, leading Harry on a new adventure to uncover the truth about his family history. This might involve exploring old wizarding families, ancient spells, and long - forgotten secrets. It could also lead to a power struggle within the wizarding world as different factions react differently to Harry's new identity.