Obviously, Harry and Hermione are the main characters. They are likely to be older, more mature versions of themselves. Harry, with his Gryffindor courage still intact, and Hermione, with her vast knowledge of magic.
Well, it could be about their continued adventures in the wizarding world. Maybe they are working together on some new spells or dealing with a new dark force that has emerged. They might also be mentoring the new generation of wizards at Hogwarts.
One common theme is family. Since it's 19 years later, they likely have a family of their own, so stories could revolve around family values, sibling relationships among their children, and how they pass on the traditions of the wizarding world to the next generation.
Another possibility is that it could focus on their adventures together. They might go on some sort of mission or exploration within the wizarding world. It could be related to a new threat that has emerged, and they have to use their combined skills and knowledge to deal with it.
It could be about Harry, Ron and Hermione's careers. Maybe Harry is now a high - ranking Auror, leading teams to fight the remaining dark wizards. Ron could be running Weasley's Wizard Wheezes successfully, and Hermione might be making great changes in the Ministry of Magic with her intelligence.
The main characters are often the children of the original Harry Potter characters. For example, Albus Severus Potter, James Potter, and Lily Potter. Also, Scorpius Malfoy is usually a central character as he is part of the new generation at Hogwarts.
In '19 years later' in Harry Potter fanfiction, we might see the next generation of wizards at Hogwarts. Harry's kids could be getting into all sorts of adventures, just like he did. Maybe they'd be facing new threats to the wizarding world.
The children of Harry, Ron, and Hermione would likely be main characters. For instance, Harry's son could inherit his father's courage and be at the center of a new mystery. He might have to deal with the high expectations placed on him because of his father's legacy.
One possible storyline could be the children of the original characters starting their own adventures at Hogwarts. They might face new threats that are related to the past but in a different way. For example, a new dark wizard could be trying to revive the ideas of Voldemort and the kids have to stop it using the knowledge passed down from their parents.
Hermione Granger would be a likely main character. She has always been a central figure. Maybe she's working on some revolutionary magical research or is involved in political reforms in the wizarding world. Her relationship with Ron and their family life could also be part of the story.