There might be a fullbring Naruto fanfiction where the concept of fullbring is incorporated into the Ninja world in a unique way. For example, fullbring abilities could be related to the different chakra natures. So Naruto, with his wind chakra, could have a wind - based fullbring that allows him to manipulate air currents in ways he couldn't before. This could lead to some really cool battle scenes and new strategies.
One interesting Naruto fullbring fanfiction could be one where Naruto discovers a new form of fullbring that combines his chakra powers with it in a unique way. For example, he might be able to fullbring out the essence of the Nine - Tails in a more controlled and powerful form.
Naruto himself is an obvious choice. His ability to adapt and his large amount of chakra could lead to some really cool fullbring combinations. He could bring out new and powerful forms that we've never seen before.
One possible plotline could be Naruto discovers he has a hidden Fullbring ability. He then has to learn to control it while still dealing with the threats in his own world. Maybe he uses it to fight stronger enemies like Madara in a new and unexpected way.
Fullbring might make Naruto more empathetic. If his fullbring is connected to someone else's emotions or memories (which is possible in some interpretations of fullbring), he would gain a deeper understanding of others. This could change his relationships with his friends and enemies alike. For instance, he might be more forgiving towards his foes if he can sense the pain that led them to their actions through his fullbring.
I'm not sure about a specific 'naruto has a fullbring' fanfiction off the top of my head. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Naruto fanfictions and you might be able to find one with this concept there.
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.