Well, a common theme in fanfiction about them could be a battle against a powerful, dark force. Solgaleo and Lunala are the heroes that rise to the occasion. Their combined strength is unbeatable.
One common theme could be a romantic relationship involving Lunala. Since it's a 'lemon' fanfic, there might be elements of attraction and love between characters. Another theme could be the exploration of Lunala's unique abilities within the context of the story.
A common theme is empowerment. Since Lunala is a powerful Pokémon, in 'female Lunala fanfiction', she often uses her powers to overcome obstacles. Another theme is friendship. She might form bonds with other Pokémon or trainers. For example, she could team up with a trainer on a quest.
In some fanfictions, Solgaleo and Lunala are depicted as having their own personalities. Solgaleo might be a noble and fearless protector, while Lunala could be more of a mysterious and wise being. A fanfiction could explore their relationship with each other and how they interact with the humans in their world. Maybe they choose a special trainer to help them in a quest that is crucial for the balance of the Pokemon universe.
There might be a 'pokedex horror story' where the Pokedex starts to give out strange and menacing messages about an upcoming disaster related to Pokémon. It warns the trainer but no one else believes it, creating a spooky situation. The trainer has to figure out if it's a real threat or just a malfunction while being constantly on edge.
The 'pokedex horror story' might be about glitches in the Pokedex system that lead to terrifying situations. For example, it could show Pokémon that are not supposed to exist or have distorted appearances. It could also be about a Pokedex that somehow predicts doom or danger related to certain Pokémon.
One of the popular characters might be the main Pokedex holder, who is often depicted as brave and curious. Another could be a rival Pokedex holder, adding some competition and drama to the story. Also, some of the Gym Leaders or Elite Four members from the original Pokemon series might make appearances and be popular in these fanfics as well.
Misty is also a character that shows up a lot. Her knowledge of water - type Pokemon can be combined with her role as a Pokedex holder. Fanfictions might explore her adventures in different regions, her encounters with new Pokemon, and how she uses the Pokedex to learn more about them.
I'm not sure specifically which Lunala Lemon fanfics are good. You could try searching on fanfic platforms like Archive of Our Own. There might be various stories there that feature Lunala in interesting ways, though I can't guarantee the quality of every single one.