I really like 'Thranduil and the Human Healer'. In this story, a human healer comes to the forest realm. Thranduil is initially skeptical but then grows to respect and maybe even have deeper feelings for the healer. There are also elements of adventure as the healer has to deal with threats to the realm while trying to understand the elven ways.
Well, there's 'Love Beyond Species: Thranduil and the Human'. This fanfiction focuses on the relationship that develops between Thranduil and a human character. It's interesting as it explores how their different natures interact. Also, 'The Human in Thranduil's Court' is a good read. It shows how a human adapts to the elven court life under Thranduil's rule.
One common theme is the clash of cultures. Since Thranduil is an elf and the other character is human, their different ways of life, values, and beliefs often collide. For example, elves might value nature in a more spiritual way compared to humans.
One great fem Harry and Thranduil fanfiction is 'A Different Path'. It explores a unique relationship between the two characters where fem Harry brings a new perspective to the Elvenking's world. The story is well - written with vivid descriptions of Middle - earth and Hogwarts elements combined in an interesting way.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). There are numerous works there. Just use the search function and type in 'Thranduil x Human Reader' and you'll get a whole list of stories. You can also check out Fanfiction.net, though the selection might be a bit different.
A prevalent theme is the exploration of different worlds. The human reader enters the elven realm and has to adapt to its beauty and mystery. There's also the theme of sacrifice. Sometimes the human might sacrifice their own world or future for Thranduil, or vice versa. And of course, there's always the theme of love's power to bridge the gap between two very different beings.
There's also 'Thranduil and the Lost Soul'. In this fanfiction, an OC who is a lost soul finds their way into Thranduil's domain. The story delves deep into the lore of the elves and how Thranduil deals with this unexpected visitor. It's a great read if you like stories that explore different cultures within Middle - earth.
There's a fanfiction called 'Thranduil's Encounter with the Dragon King'. In this story, Thranduil has to face a powerful dragon king and the interactions between them are full of tension and mystery. There are also some stories on fanfiction.net that are really good, you can search there using the tags 'Thranduil' and 'dragon' to find more.
Another place to look is Archive of Our Own (AO3). There are a lot of talented writers in the Tolkien fandom there. A good fanfiction might involve Thranduil having to protect his people from a dragon's fire. Maybe the dragon has been awakened by some dark magic and Thranduil is the only one who can stop it. The fanfiction could explore his character development as he overcomes this great threat.
I also like 'Thranduil's Unforeseen Love'. This fanfiction takes an interesting approach where Thranduil falls for a human who has come to seek his help. It explores the difficulties of their relationship due to the differences between elves and humans, but also the power of love that overcomes them.
One great Harry and Thranduil fanfiction is 'The Meeting of Two Worlds'. It explores an alternate universe where Harry somehow ends up in Thranduil's realm. The story beautifully weaves the magic of the Harry Potter universe with the elven world of Thranduil. It has some really interesting character interactions, especially when Harry tries to understand the elven culture.
I'm not sure which specific 'Thranduil lemon fanfiction' is good as it often involves non - canonical and sometimes not - so - mainstream interpretations. However, you can try looking on fanfiction - dedicated websites like Archive of Our Own. Just search for 'Thranduil' in the search bar and browse through the results to find stories that might interest you.