Perhaps there's a magical or otherworldly force at play in the fanfiction. For example, a powerful witch or wizard discovers that Brightburn is a threat to the fabric of their world. They use ancient spells and incantations to bind him. Once he's immobilized, they find a way to shatter his physical form, maybe by using a magically enhanced weapon. This would be a very different take from the more science - based scenarios, but in the realm of fanfiction, the sky's the limit.
Well, in a fanfiction, he might be killed by his own hubris. Let's say he continuously underestimates the power of human ingenuity. A group of scientists manage to create a device that disrupts his energy source. While he's weakened, the military launches a full - scale attack using advanced weaponry specifically designed to target his unique physiology. So, his overconfidence in his invincibility leads to his downfall.
One possible scenario could be that Natsu is killed in a battle against a powerful and unexpected enemy. Maybe a new dark mage appears with powers that can overpower Natsu's fire magic.
One possible scenario could be that he is killed in a lightsaber duel by a powerful Sith opponent. Maybe this new Sith has some secret technique that Anakin couldn't counter.
In such a fanfiction, one possible scenario could be that Gladion gets involved in a battle against a powerful and malicious Pokemon trainer. The trainer's Pokemon might use a super - powerful attack that Gladion's Pokemon can't withstand, and in an attempt to protect his Pokemon, Gladion gets caught in the crossfire and is killed. Another scenario could be that he stumbles upon a dangerous, secret organization within the Pokemon world that doesn't want their secrets exposed. They might target Gladion and assassinate him to keep him silent.
It could be his own mother. She might realize that he has become too dangerous and, in a moment of heart - breaking decision, find a way to destroy him. Maybe she discovers a weakness and uses it against him, driven by her love for humanity and the knowledge of the destruction he could cause.
One possible scenario could be a power imbalance. Maybe Sasuke discovers some new, extremely powerful jutsu that Naruto has no defense against. For example, a jutsu that can nullify Naruto's chakra or attack his spirit directly. Another possibility is a miscommunication or a breakdown in their relationship. If Sasuke believes Naruto has betrayed him in some way, his anger could lead him to kill Naruto. However, this goes against the core of their bond in the original series.
In a more psychological approach, Naruto could have grown so much as a character that he no longer fears Kakashi's reputation. This new - found confidence could change the dynamics of their fight. He might be able to read Kakashi's moves better and anticipate his attacks. Combined with his own ever - improving skills, Naruto could defeat Kakashi in this state of mental superiority.
One scenario could be when Naruto finds out that someone has been spreading false rumors about his friends. This would surely make him pissed as he values his friends deeply.
In the fanfiction, perhaps there's a new recruit in the Joker's gang who is very talented and starts getting a lot of attention. The Joker, being used to being the center of attention, gets jealous. He might start giving the new recruit impossible tasks to do in order to undermine them and regain the spotlight.
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.
In 'annabeth gets hurt fanfiction', she may get hurt while trying to solve a mystery. Let's say she's investigating an old temple and there are booby - traps. One of the traps might shoot out poisoned darts and she gets hit. This then forces her friends to find a cure quickly. Also, she could be in a fight with a rival group and they use some underhanded tactics that result in her getting hurt, like a sneak attack that injures her leg.