A theme that is often seen is the idea of adapting to new situations. The Scooby Gang has to quickly get used to the magic around them, and the Harry Potter characters have to deal with the Scooby Gang's unorthodox methods of investigation. This creates a lot of interesting scenarios where characters grow and learn from each other as they try to adapt to the new circumstances they find themselves in.
One great 'Harry Potter Fanfiction Scooby Gang Fanfic' could be 'The Mystery at Hogwarts'. In this fic, the Scooby Gang somehow finds themselves transported to Hogwarts. They team up with Harry and his friends to solve a mystery that involves both magic and some spooky elements that the Scooby Gang is used to. It's really interesting to see how the two groups interact and how the Scooby Gang's detective skills are put to use in the magical world.
One more common theme is the struggle against dark forces. Even in fanfiction, the gang members often find themselves up against some form of evil. It could be a new dark wizard or a dark creature. They have to use their wits and magic to overcome these threats. Also, the theme of growing up is present. We see the characters evolving from mischievous students to more responsible young wizards as the story progresses.
One possible goal could be to protect the wizarding world from new and unknown threats. Since Harry has always been a defender, his gang would follow suit. Another goal might be to promote unity among different wizarding groups. In the original series, there were some divides, but as a leader, Harry could aim to bridge those gaps through his gang.
One common theme is adventure. Since they are from a mystery - solving gang, fanfics often involve them going on new adventures together. Another theme is the discovery of their feelings for each other. Usually, they start as friends in the original show and fanfics explore how those feelings change.
You'll often find Draco Malfoy in these fanfictions too. He could be the leader of a rival gang, with his Slytherin cronies. Neville Longbottom might also be a character, perhaps having transformed into a tough and resourceful member of a gang, using his knowledge of Herbology to create magical defenses for the gang. And of course, Dumbledore could be a sort of wise figure in the background, guiding Harry or other protagonists from afar.
Harry's gang could include some of the characters we don't see much of in the original books. For example, the house - elves. Dobby was already very loyal to Harry. Maybe he could convince other house - elves to help out in a more active way. There could also be some of the older students who dropped out of Hogwarts for various reasons but still have a lot of magical knowledge. And of course, Hagrid could be an honorary member, providing his vast knowledge of magical creatures and his big - hearted support for Harry and the gang.
Loyalty is another big one. It shows how they stick together through thick and thin in the dangerous gang world. For example, in many fanfics, a member might face a difficult situation but the others will not abandon him because of their loyalty.
One common theme is friendship. Their bond is always a central focus, like in many fics where they support each other through tough times. Another is adventure. Since they're from a mystery - solving gang, lots of fanfictions have them going on new adventures together.
Well, since it's rated M, it might have themes like more intense mystery elements. Maybe the villains are more menacing and the danger to the characters is more real. It could also involve more grown - up relationships among the gang, like romantic subplots that are more complex and not as simple as in the original series.
Friendship is a very common theme. Since Shaggy and Scooby have such a strong bond in the original series, fanfics often explore that deeper. Another theme is adventure. They are always going on new adventures in the fanfics, whether it's exploring a haunted castle or a mysterious island.