Goku can be developed by giving him more complex emotions. In fanfiction, he could struggle with feelings of jealousy if Sanzo spends more time with another character. This would not only add depth to Goku's character but also test his relationship with Sanzo.
Another way to develop the characters is to put them in situations where they have to make difficult choices. For instance, if they have to choose between saving a village or continuing on their journey. Sanzo's decision - making process based on his beliefs and Goku's more instinct - based decisions would highlight their differences and how they grow from these experiences.
One idea could be exploring a time when Sanzo loses his memory and Goku has to help him remember who he is and their mission. This would create a lot of emotional moments as Goku tries to jog Sanzo's memory with their shared experiences.
You can start by looking on fanfiction.net. It has a large collection of various fanfics, including those related to sanzo x goku. There are often tags and categories that can help you narrow down your search.
One way is to focus on their shared adventures. For example, when they are on a quest for a new power source or to stop a new threat, they can rely on each other more. Goku can protect Bulma physically, while Bulma can use her intelligence to help Goku strategize. Another way is to explore their past more deeply. Maybe there are some untold stories from when they first met that can strengthen their bond.
For Gay Sabastion, you could explore his backstory more. Maybe he had a difficult childhood that made him the person he is today. This could add depth to his character and his relationship with Ciel.
For Mallow, her character could be developed further by exploring her family background more deeply. Maybe there are secrets in her family related to a certain type of Pokemon. In the context of Pokemon Sun, the main character could help her uncover these secrets, which would make her more confident and also add more depth to her character.
For 2D, you could explore his insecurities more deeply. Maybe he's insecure about his voice compared to Ace's. So, he starts taking vocal lessons from Ace in the fanfiction. This would not only develop his character but also show his determination to grow as an artist.
To develop Miketsukami's character further, we could explore his past more deeply. Maybe there are some hidden traumas or experiences that have shaped his loyalty towards Ririchiyo. For Ririchiyo, we can show her growing into a more confident leader, using her relationship with Miketsukami as a source of strength.
With Reborn, you can develop his past. Maybe write a story where we learn more about his training and how he became the world - class hitman. This could add more depth to his character and also show how his past experiences influence his relationship with Tsuna. For example, if he had a difficult master - apprentice relationship in the past, it could make his bond with Tsuna even more special.
For Knuckles, you could explore his relationship with his tribe more deeply. Maybe in the fanfiction, he discovers some ancient scrolls left by his ancestors that give him new powers or knowledge. This could change his interactions with Rouge and Sonic. As for Rouge, you could develop her character by showing more of her backstory, like how she became interested in jewels and espionage in the first place.
To develop Goku Jr's character further, he could be given more complex emotions. For example, showing his fear in the face of truly overwhelming enemies, rather than always being fearless like Goku. This would make him more relatable. Also, exploring his relationships in more depth, like his friendship with Pan. Maybe they have a falling out over different views on how to protect their world, which forces Goku Jr. to grow and understand different perspectives.