A common theme could be friendship. Since Fred and Shaggy are friends in the original context, fanfics might explore a deeper level of that friendship, maybe with some added emotional elements.
Adventure is another theme. Fans might write about Fred and Shaggy going on new adventures together that are not part of the original story. They could face new challenges and overcome them as a team, with their relationship evolving along the way. This can add an exciting new dimension to the pairing.
Overcoming fears can also be a theme. Shaggy is known for being a bit of a scaredy - cat, and in fanfiction, Fred might help him face his fears, or vice versa. For instance, in a story where they have to face a powerful supernatural being, they encourage each other to be brave and find a way to defeat it.
In 'fred and shaggy gay fanfiction', a frequent theme is probably the exploration of their relationship in different settings. For example, how they interact in a world where their relationship is accepted or in a more secretive situation. Also, themes of loyalty and support within their relationship are likely to be common, as they face challenges together. It could also involve themes of acceptance, both from themselves and from their friends in the story.
A frequent theme is the growth of their relationship. It often starts from a friendship and then slowly blossoms into something more. Also, there's the theme of support. In many fanfics, they support each other through different challenges, whether it's a mystery they are solving or personal issues. And sometimes, there's the theme of secrecy, as they might try to hide their new relationship from the rest of the gang at first.
One common theme is friendship exploration. Since they are part of the Scooby - Doo gang, many fanfics look at how their friendship could change and grow deeper. Another theme is mystery - solving together in a more personal way, not just the typical Scooby - Doo mysteries.
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.
Love and romance are very common. Writers often explore the relationship between Fred and Ginger, how they fall in love, and their journey together.
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.
One common theme is the exploration of their hidden feelings. Since they've been friends for a long time in the original series, fanfiction often plays with the idea of them realizing there's more to their relationship.
I'm not sure of specific 'fred x shaggy fanfic' titles off - hand. But often, if you join relevant fan communities on social media platforms like Reddit or Tumblr, fellow fans can point you towards some good ones. They might have personal favorites that they are willing to share.
There might be a fanfiction where Shaggy and Fred have an argument at first, but then through a series of events, they come to understand each other better. For example, they could be competing for the same role in a school play or something similar, and in the process, realize their true feelings for one another. It could be a story full of drama and character growth.