Well, in some fanfictions, Naruto's dense muscles are often related to his unique chakra control. Since he has so much chakra, his body might have developed dense muscles to handle it better. This could also be part of the reason he's able to perform some of his more powerful jutsu without overexerting himself physically.
One common theme is power enhancement. With the added muscle, Naruto is often portrayed as having greater physical power, which can translate into more effective combat abilities. For instance, his taijutsu could be on a whole new level.
Naruto's dense chakra could make his friends either jealous or in awe. If he can do things with his chakra that others can't, some might envy his power. But his true friends like Sasuke and Sakura would be in awe and see it as a way to strengthen their team. For example, Sasuke might see Naruto's dense chakra as a new challenge to keep up with, which could improve their friendly rivalry.
One interesting aspect of Naruto having muscles in fanfiction could be how it changes his appearance and how other characters react. Some might be in awe of his new, more muscular look, while others might be a bit intimidated. It could also lead to new rivalries or friendships based on this change in his physique.
One common theme is the reaction of others. Characters in the 'naruto' universe will react to Naruto's new muscular form. Some may be jealous, some may be inspired. Also, the training process is often a theme. How Naruto endures tough training regimens to achieve his muscular physique is detailed in many fanfictions. In addition, there's the theme of using his new - found strength in battles against various enemies, which showcases his growth as a character.
One interesting plot could be a story where Naruto discovers a new form of chakra that is extremely dense and he has to learn to control it. This could lead to him facing new and powerful enemies. Another plot might involve Naruto sharing his dense chakra with his friends to help them in battles. For example, he could transfer some of it to Sakura to enhance her medical ninjutsu or to Sasuke to boost his Sharingan powers.
Power dynamics is also a theme. His muscles can give him an edge in physical confrontations, which changes the power balance in his interactions. For example, in some fanfics, it makes the Slytherins more wary of him, as they see him as a more formidable opponent.
One common theme is self - discovery. When Merlin has muscles, he often has to re - evaluate his own identity and how he presents himself. For example, he might be seen as more of a warrior type, which is different from his traditional role as a servant and a secret sorcerer.
One great 'Merlin has muscles' fanfiction is 'Muscular Merlin's Adventures' on FanFiction.net. It really focuses on how Merlin's newfound muscles change his dynamic with Arthur and the knights. It has some really interesting character interactions and a unique take on Merlin's character development.