One common theme is power dynamics. Since Thranduil is a powerful elf king and dragons are also very powerful creatures, their interactions often involve a struggle for power or respect. Another theme is mystery. Dragons are often mysterious in these stories, and Thranduil tries to figure them out.
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.
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.
It could be that there was some sort of magic involved. Maybe an enemy of Thranduil cast a spell that made the dragon more aggressive towards him. The dragon fire might have some special properties due to this magic, and Thranduil's burns could be not just physical but also have some sort of curse or negative effect on his being. He then has to find a way to break the curse while dealing with his injuries.
One interesting fanfiction could be where Thranduil uses his elven magic to counter dragon fire. Maybe he creates a shield of ancient elven power that not only withstands the intense heat but also reflects the fire back at the dragon. This would show his great power and wisdom as an elven king.
Thranduil can be developed by showing his vulnerability. With the OC being pregnant, he might worry about losing her or the unborn child. This could make him more relatable. For example, he might show a side of himself that is less regal and more human - like in his concerns. Another way is through his interactions with the OC. He could be more affectionate, like gently touching her belly and sharing his hopes for the future with her.
There's also 'The Elven Heart'. This story focuses on a time when Tauriel is injured and Thranduil must care for her. It explores their feelings for each other that they have long kept hidden. The dialogue is well - written and gives a lot of insight into their characters.
In RWBY fanfiction, King Thranduil can be a really interesting addition. He could bring his elven grace and combat skills. For example, he might form an alliance with the RWBY characters. His leadership and wisdom could be a great asset in the fight against the Grimm. Also, his unique appearance would surely stand out in the RWBY universe.
King Thranduil can play the role of a powerful ally. He has his own army and resources which could be useful for the RWBY characters. For instance, his archers could help in battles against the Grimm.
Most Thranduil love fanfiction involves beautiful descriptions of the elven realm. It's like the love story is set against this backdrop of a magical, forest - filled world. The love is also often depicted as something that is both forbidden and irresistible, adding an element of drama.