Set the fanfic in an unusual or unique setting. Instead of the typical urban or fantasy settings, put them in a post - apocalyptic world where resources are scarce. The guy has to not only protect the girl from other humans who might be desperate, but also from the harsh environment.
Create a complex backstory for both the guy and the girl. If the guy has a past trauma related to not being able to protect someone before, it adds depth to his need to protect the girl. And if the girl has a secret that makes her a target, it makes the protection more crucial.
Add unexpected twists. For example, the girl could turn out to have some hidden power that she reveals while being protected, which then helps the guy in turn. This flips the dynamic a bit.
You could introduce some conflict. Maybe the guy is looking at the girl, but there's another guy who also has his eyes on her and is a bit of a rival. This creates tension and makes the story more engaging.
Create a backstory for the girl. Maybe she has a special ability or a past trauma that makes her reaction to the monster's lick more unique. It could also add depth to the story and make readers more engaged.
By making it all about a misunderstanding. For example, the girl might be a scientist doing an experiment that goes wrong and affects the guy's bladder in an unexpected way. The story can focus on how they work together to reverse the effect, with no malicious intent from the girl and a lot of humor in the situation.
Incorporate time - travel elements. With his immortality, Naruto could be sent back in time to change certain events in the Naruto universe. This would open up a whole new world of possibilities, like preventing some of the major tragedies that occurred in the original story or changing the fates of certain characters.
Involve other characters in unique ways. Maybe Aladdin could be a rival for Jasmine's affections in a different way than in the original story. Or have the Genie meddle in Jafar and Jasmine's relationship, perhaps trying to nudge Jafar towards the good side for Jasmine's sake. These character interactions can create more complex storylines.
One popular 'guy protects girl' fanfic could be in the 'Twilight' fandom. In many fan - made stories, Edward might be protecting Bella in different, more creative scenarios than in the original books. For example, protecting her from new supernatural threats in Forks.
By adding mystery. For example, the snake could have a hidden power that is only unlocked when it squeezes the girl. Maybe it can transfer memories or special abilities to her.
You can make it interesting by adding some mystery. For instance, the big dog may have a strange mark on its fur and every time it licks the girl, she has a strange vision. This will keep the readers guessing.
To make it more interesting, you could introduce a new and mysterious character that Percy meets while alone. This character could have a hidden agenda or a special connection to Percy's past.
By creating a unique and detailed setting. Say it's set in a post - apocalyptic world where resources are scarce and wearing diapers is a strange form of survival mechanism. Also, adding some mystery elements, like who is behind forcing this on the characters and what their motives are, can make it more interesting.