Harry Potter himself could be the main character. Given his history with Voldemort and his Death Eaters, it would be interesting to see him try to get closer to an enemy to gain an upper hand in the fight against the Dark Arts.
Perhaps the fanfiction could explore how keeping an enemy closer affects the character's relationships with their friends. They might have to keep secrets from their friends, which could lead to some tense and interesting situations. For example, if Harry Potter were to befriend Draco Malfoy secretly, Ron and Hermione would be very suspicious and it could create a rift in their trio.
Voldemort and his Death Eaters would still be the main enemies. But now, Harry's guns would pose a new threat to them. They might not be used to dealing with such Muggle - like weapons, even if they are magically enhanced.
One possible theme could be family. Since it's about being closer to home, family relationships, whether good or bad, might be explored. Another theme could be the contrast between the magical world and the normal world at home.
Voldemort or his remaining Death Eaters could be the main enemies. They would see Harry's new blade master abilities as a threat. Another potential enemy could be a dark wizard from a foreign land who is jealous of Harry's power.
One common theme is the discovery of hidden enemies. In many fanfictions, there are characters who seem friendly at first but are later revealed to be enemies. Another theme is the power struggle between Harry and his enemies. It often involves magic battles and strategic planning. Revenge is also a theme, as some enemies seek revenge on Harry or his friends for past actions.
Alien races that are envious of the Earth - born magic could be enemies. They might see Harry and the wizards as a threat to their own power and dominance in the universe. Also, there could be dark wizards who have escaped to space to avoid the law on Earth and continue their evil deeds, now with a new set of space - related challenges and opportunities.
Voldemort would still be a major enemy. But with Kratos raising Harry, Voldemort might face a much more challenging opponent. Also, any dark wizards or creatures that oppose Harry would be in trouble. Kratos wouldn't tolerate any threats to Harry, so even the Death Eaters would have to be extra careful. Another potential enemy could be those in the wizarding world who oppose Kratos' methods of raising Harry, seeing them as too brutal or un - wizard - like.
Some of the dark wizards who are looking for a new source of power might target Harry. They might think that by capturing or controlling him, they can access the powers of Trigon. There could also be entities in other dimensions that see Harry as a key to opening portals between worlds and want to use him for their own malicious purposes.
Well, it might explore the characters in a more personal or familiar setting. Maybe it shows the Hogwarts students in a situation closer to home, like during their holidays. It could also focus on the relationships they have with their families which we don't often see in the original books.