Another common theme could be the exploration of power dynamics in a relationship. Given Piper McLean's own powers and abilities, how does that play into a romantic relationship? Does it create tension or does it bring them closer? And in the context of 'lemon' fanfiction, this power dynamic can be explored in a more intimate and sometimes even a bit more risqué way, though not always crossing into inappropriate territory.
One common theme is friendship. Since they come from different worlds, their growing friendship is often explored.
The theme of self - discovery is quite common. As they are in a relationship, they also discover more about themselves. Love against all odds is also a prevalent theme. In many fanfics, they face challenges like opposition from other characters or difficult situations, but their love prevails. Additionally, the theme of power and how they use it in relation to their relationship is often explored. For example, how their powers complement each other or cause some friction at times.
One common theme is their relationship development. Many fanfics explore how they fall in love, deal with their differences, and grow together. Another theme is their adventures outside the main Percy Jackson storyline. Writers like to take them on new quests and challenges.
A theme that also pops up a lot is the discovery of new powers or abilities. Piper might discover a new aspect of her charm - speaking, and Percy could find a new way to use his water powers. Also, the theme of family and heritage is common. They both come from complex backgrounds as demigods, and fanfictions often dig deeper into how that affects them and their relationship.
Some fanfics might also have a theme of power dynamics. Annabeth and Piper have different powers and personalities in the original series. In the lemon fanfics, this can be explored in terms of how they interact with each other in more intimate situations. For example, how their leadership qualities or magical abilities play into their relationship.
Since it's a crossover, it combines elements from both Naruto and Piper McLean's universes. However, as it's a 'lemon' fanfiction, it may involve romantic or sexual undertones between the two characters which might be a bit out of the ordinary given their different origins. But in general, it would probably play with their powers and personalities in a new context.
Friendship is a very common theme. Since Percy and Piper are both demigods, their friendship is often explored as they face various challenges together.
Another theme is sacrifice. Given their heroic natures, fanfictions may explore how they are willing to sacrifice for each other in the face of danger. This could be sacrificing a chance at power or even their own safety for the sake of the other.
Another one is 'Under the Stars'. In this story, Piper and Leo find themselves on an unexpected adventure together. The author does an excellent job of portraying their personalities. The story is filled with action, a bit of mystery, and of course, their special bond that makes it a must - read for fans of their pairing.
I'm not sure specifically which Piper McLean lemon fanfictions are the best. You can try searching on popular fanfiction platforms like Archive of Our Own. They usually have a wide range of fanfictions related to different characters, and you might find some interesting Piper McLean - related ones there.