One common theme could be the exploration of the extreme contrast between the two species. The xenomorph is a terrifying, alien creature, while humans are more relatable. In fanfiction, this contrast might be used to create intense and unique relationships. Another theme could be survival in the face of the xenomorph's threat, with the human characters having to use their wits and any possible alliances, even with a xenomorph in some cases, to stay alive. Also, the idea of transformation or adaptation might be present, perhaps a human character taking on some traits of the xenomorph or vice versa in this fictional and often strange world of fanfiction.
It could start with fear and curiosity. The human is initially terrified of the xenomorph but as they observe it, they notice some unique behaviors. This piques their curiosity and they start to approach it cautiously.
Well, a 'xenomorph and human lemon fanfic' is probably a very unique piece of fan - created work. 'Lemon' in fanfic terms can sometimes mean a story with some sort of romantic or at least very close relationship undertones. But when you combine a human with a xenomorph, which is a terrifying alien creature, it creates a really strange juxtaposition. It could be a story about how two very different beings interact, perhaps in a world where the normal rules of biology and relationships are bent or broken. Maybe it's a story of an unlikely friendship or a more complex relationship that challenges our understanding of what is possible between such different entities.
It could be a dark wizard who has somehow managed to control the Xenomorphs and is using them to cause chaos. This wizard would see Harry as a threat to his plans and would constantly send the Xenomorphs after him, creating those 'lemon' - like difficult and dangerous situations for Harry.
One possible plot could be that Harry, through some magical accident, ends up in a world where Xenomorphs exist. He uses his magic to try and understand them, and perhaps forms an unexpected bond with a young Xenomorph that's different from the rest. The 'lemon' aspect could be his initial fear and the adrenaline - filled encounters that are like a sour 'lemon' taste, but eventually lead to something more positive.
I'm not really into that specific type of fanfiction, but you can try searching on some dedicated fanfiction websites like Archive of Our Own. There might be some 'human x xenomorph' stories, though I'm not sure about the 'lemon' aspect as it might involve adult - like content which is not always easy to find or appropriate.
There might be some out there, but a lot of such fanfiction can be quite niche and hard to find. You could start by looking at the larger fanfiction communities related to the Xenomorph series. However, keep in mind that the 'lemon' aspect often involves mature or inappropriate content in many cases, so it might not be something that is widely promoted or easy to access in a more mainstream way.
One idea could be a crossover where Naruto characters encounter Xenomorphs in a hidden part of their world. For example, the Xenomorphs could be brought to the Naruto universe through a mysterious jutsu gone wrong. Naruto and his friends would then have to use their ninja skills to fight these deadly creatures. Maybe Sasuke could use his Sharingan to predict the Xenomorphs' movements.
Maybe the Xenomorph Queen herself could be the main antagonist. She is intelligent and powerful. She sees Naruto's world as a new place to colonize and sends her Xenomorph minions to attack. Naruto has to find a way to stop her before she spreads too many eggs and creates an unstoppable army.
One possible plot could be that Naruto discovers a hidden Xenomorph lair in the Hidden Leaf Village. He uses his ninja skills to try and figure out how the Xenomorphs got there and what their purpose is. Maybe he has to team up with some of his friends, like Sasuke, to fight off the Xenomorphs before they take over the village.