Friendship is a very common theme. Their unique friendship in the show often gets explored further in fanfics. For example, stories might focus on how they support each other through tough times at school or in their family.
A plot could revolve around Gumball and Darwin getting a lemon as a gift. This lemon is not an ordinary one but has a secret message inside it. They have to figure out how to read the message. Along the way, they face some challenges like a bully who wants to steal the lemon from them. They use their friendship and wits to overcome these obstacles and finally uncover the message which leads them to a new discovery in their town.
The characters' relationship could be developed through shared experiences with the lemon. For example, if they find a lemon that has a special power only they can understand, they'll have to rely on each other more, which would strengthen their bond.
One idea could be a story where Gumball and Darwin get lost in a magical forest and have to rely on their friendship to find their way back home. They could encounter strange creatures and solve puzzles together.
I'm not sure if there is such specific 'gumball x darwin human lemon fanfiction' available. Fanfiction can be quite diverse and often depends on what different writers have created. It might be a rather niche combination, so it could be hard to find.
Perhaps a fanfiction where they get lost in a magical forest. They meet strange creatures and have to work together to find their way back home. Along the way, they learn more about their friendship and themselves.
You can try fanfiction websites like Archive of Our Own (AO3). There are often a lot of great Tawog Gumball x Darwin fanfics there. Just search for the relevant tags and you'll be presented with a wide range of stories.
Begin with an interesting hook. For example, you could start with a strange event that involves both Gumball and Darwin, like 'Gumball and Darwin woke up one morning to find that their house had turned into a giant bubble.' This will draw the reader in right away. After that, introduce the characters' reactions and start building the story from there. Also, don't be afraid to add your own unique twists to their personalities or the world they live in.