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.
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.
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.
There are quite a few. You can start by looking on fanfiction websites like Archive of Our Own. Some might focus on their adventures together in a new and exciting setting, like exploring a mysterious haunted house where their friendship is put to the test and they have to rely on each other to get out.
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.
Well, I've come across some really interesting Scooby Doo Fred x Shaggy fanfictions. One that stands out had them going on a side adventure separate from the rest of the gang. It delved into their personalities, how Fred's leadership skills and Shaggy's more laid - back nature interacted in a unique situation. They faced a mystery that was more personal to them and through it, their bond grew even stronger. There are many others that explore different aspects of their relationship as well.
Some fandom - specific forums might also be a good source. Fans often share their own works or recommend good fanfictions in these forums. You can search for Scooby - Doo related forums and see if there are any posts about Shaggy x Fred fanfiction.
The popularity might come from the unique dynamic between them. Fred is the leader, and Shaggy is more of a scaredy - cat. Their differences make for interesting storylines.
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.
Well, they could start off as reluctant allies. Shrek might not trust Shaggy at first because he seems so skittish, and Shaggy is afraid of Shrek's appearance. But as they face challenges together, like fighting off a common enemy, they begin to respect each other. For example, when a group of witches tries to capture them both for their own evil plans, they have to rely on each other's unique skills. Shrek's brute strength and Shaggy's quick thinking and ability to find hiding places.