One common scenario is when his friends are in danger. Goku is extremely protective of his friends, and if he sees them being seriously threatened or hurt, he'll get mad. Another is when an enemy disrespects his family or the Saiyan race. Goku has a strong sense of pride in his heritage, and any slights against it can enrage him.
In DBZ fanfiction, often when an enemy targets the Earth or any of the innocent people on it, Goku gets mad. He has a strong sense of justice and protecting the weak. Also, if an enemy cheats in a fight or uses underhanded tactics against him or his friends, that'll make Goku angry. He believes in a fair fight and any violation of that principle pisses him off.
Well, perhaps in the fanfiction, Goku gets mad when he discovers that someone has been using his name for evil deeds. This would go against his values of justice and protecting the innocent. He might get really angry and go on a search to clear his name and stop the impostor. Or it could be that a new, extremely powerful villain emerges and starts to destroy planets without any remorse. Goku, being the defender of the universe, would be enraged by such senseless destruction and would confront the villain with all his might.
One common theme is the growth of Gohan. Goku often plays a role in pushing Gohan to reach new heights in his power and self - discovery.
One common theme is rivalry - turned - friendship. Their initial intense rivalry gradually transforms into a deep bond as they face various threats. Another theme is power - up. They often find new ways to increase their power together, like in some fanfictions where they discover a new fusion technique.
In some 'Goku meets Goku fanfiction', they could team up against a super powerful enemy that neither could defeat alone. They would combine their signature moves like the Kamehameha for an ultimate attack.
A third scenario could be jealousy. If other witches start showing interest in Harry and he doesn't immediately and firmly shut it down, Ginny might get mad. She is a proud witch and doesn't want to share Harry's affections, and his seeming indecisiveness in such situations could lead to her anger in fanfiction.
It could be Vegeta. Maybe Vegeta does something really underhanded like stealing Goku's senzu beans or trying to take all the credit for a victory that Goku also contributed to. Goku, being a fair - minded person, would get angry at such behavior.
In fanfiction, the unique aspect of Goku and 18 is their contrasting natures. Goku is a Saiyan, full of energy, always eager for a fight, and has a somewhat innocent view of the world. 18, on the other hand, is an android, created with a more cynical view due to her origin. Their interactions can be really interesting as they bridge these two different outlooks. For example, Goku's positive attitude might start to change 18's more negative perspective over time.
In such fanfiction, it could lead to a completely different power dynamic and relationship among the characters.
One interesting story could be about Black Goku's origin in a different universe where his transformation into an evil Saiyan had a more complex backstory involving a corrupted wish on the Dragon Balls. It might explore how his actions in that universe led to him crossing over into the main Dragon Ball Z universe.
In some 'Goku Gets Angry' fanfiction, a common scenario is when his friends are in danger. For example, if Vegeta is seriously injured by a powerful new enemy, Goku's anger flares up. He then enters a new and more powerful form to fight the foe. Another scenario could be when the Earth is threatened. Maybe a villain plans to destroy the planet, and Goku gets angry at the thought of his home and family being in jeopardy. This anger gives him the motivation to train harder and find a way to defeat the villain.