First, you need to study his character well. Know his backstory, his mannerisms, and his speech patterns. For example, his flirty yet guarded nature. Then, come up with an interesting plot. It could be something that ties into his past, like a new threat related to Cazador. Also, focus on his character development. Maybe he learns to trust more or becomes more in touch with his emotions.
One popular theme is Astarion's struggle with his vampiric nature. Many fanfics explore how he copes with his bloodlust and the moral implications. Another theme is his relationship with the main character. Whether it's a romantic or a platonic bond, writers like to delve into the dynamics between them. Also, his backstory often gets explored, like his time as a slave to Cazador and how that has shaped him into the character we see in the game.
I think it's great. The story of Baldur's Gate 3 is full of surprises and depth. It keeps you hooked from start to finish. The world-building and character development are top-notch.
Baldur's Gate 2's story is top-notch. It offers a compelling narrative that keeps you hooked from start to finish. The depth of the lore and the quality of the storytelling make it a standout in the gaming world.