Naruto himself often features prominently in such fanfiction. His already strong will and determination can be combined with the rage bloodline to create a really powerful version of him. Sasuke is also a popular choice. His brooding nature and quest for power can be intertwined with the rage bloodline in interesting ways.
Well, in some naruto rage bloodline fanfiction, there might be a story where the main character discovers that their rage bloodline has the power to manipulate the elements in a completely different way compared to normal ninjas. For instance, instead of using hand signs, they can just channel their rage to create fire or water. Also, there could be a plot where the rage bloodline is seen as a curse at first, but then the character learns to control it and turn it into an advantage, like being able to sense danger more acutely when angry.
Well, in 'naruto rage fanfiction', a frequent theme is the transformation of Naruto's character. His rage can make him more aggressive and less forgiving. Also, the theme of isolation can be present. As he becomes consumed by rage, he may push away his friends. For example, some stories show how his rage over a betrayal causes him to cut off ties with certain characters. Another theme could be redemption. He has to find a way to control his rage and redeem himself in the eyes of others or in his own eyes.
There are also fanfictions where a bloodline is created through some sort of experimental jutsu. For example, a bloodline that allows the user to transform into different elements at will. This would give the character an edge in combat as they could adapt to any situation by changing their form.
The knight bloodline could be integrated as a foreign power that was brought to the Naruto world long ago. Maybe a group of warriors from another dimension with the knight bloodline came and settled, and over time their bloodline got diluted but still resurfaces in certain individuals.
He might use some special jutsu or seals. For example, he could use a sealing technique to suppress the signs of his bloodline. This way, others wouldn't be able to detect it through normal means like sensing chakra.
It could also be part of a strategic move. By hiding his bloodline, he can surprise his opponents in battle. They would underestimate him based on not knowing about his true powers, and then he could turn the tables on them when the time is right. This gives him an edge in combat situations.
Power enhancement is a common theme. Naruto's blood rage often gives him a boost in power, allowing him to overcome tough opponents. For example, in many fanfictions, he can defeat enemies he couldn't before when the blood rage kicks in.
Another theme is the clash of cultures. In a 'naruto rage crossover fanfiction', Naruto's ninja - based culture collides with the culture of the other world he crosses into. His rage could be a result of misunderstandings or attempts to protect his own values. For instance, if he crosses into a world where honor is defined very differently, he might get angry when his ninja honor is questioned.
One common theme is revenge. Since Naruto has a lot of enemies and a tragic past in the original series, in rage - filled fanfiction, he might be driven by a desire for revenge against those who wronged him or his village. Another theme could be power. His rage might fuel his pursuit of greater power to protect himself and others. And also, there could be themes related to isolation, as his rage might make him push people away or make others fear him.