One possible plot could be that Zoro, turned into a girl, has to deal with the different reactions of his crewmates. Some might be shocked, some might tease him. And he has to learn to adapt to his new physical form while still maintaining his tough fighting spirit.
He might become withdrawn at first. Since he's used to being in control and having a certain dynamic in their relationship, Bella's anger could shock him into pulling back. He could go off on his own for a while to think about what he did wrong.
Another possibility is a jealous rival. Maybe there's someone who is jealous of Gohan's power or his relationship with certain characters. This rival could do things to annoy Gohan and push him to the edge of his temper.
A new player on an opposing team could be the cause. This new player might be overly cocky and disrespectful towards Kuroko during a game. They could constantly taunt him about his lack of presence, which finally gets on Kuroko's nerves and makes him mad.
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.
One of his friends could be the cause, accidentally. Maybe a friend makes a deal with a shady character without realizing the consequences. This deal ends up putting everyone in danger, and Yami gets mad at his friend's naivete and the mess they've created.
Well, perhaps an enemy uses a magic spell to turn Zoro into a baby as a way to weaken the Straw Hat Pirates. At first, the crew is in chaos trying to deal with a baby Zoro. But as time passes, they see that baby Zoro has a pure sense of determination. He still tries to train, albeit in a very clumsy and cute way. And in the end, they go on an adventure to find the antidote to turn him back, with lots of comical and touching moments along the way.
Definitely Doofenshmirtz. He could mess with one of Phineas and Ferb's inventions in a way that really ticks Phineas off. Like he might use one of his -inator devices to turn their amazing flying machine into a pile of junk. Phineas would be furious and would then set out to fix it and get revenge on Doofenshmirtz.
It could be his rival. Rivals in Yu - Gi - Oh are always trying to one - up each other. If his rival manages to steal a rare card that Zane has been eyeing, or if the rival taunts Zane about a recent defeat, that would surely make Zane mad.
A common theme is protecting loved ones. When Trunks sees his family or friends in harm's way, he gets mad and is driven to take action. Another could be revenge. If someone wrongs him or his loved ones, his anger fuels his desire for revenge in the fanfiction.