Well, 'Solo Leveling' also has elements of romance within its manhwa. The story mainly focuses on the protagonist's journey in a dangerous world but there are some romantic undertones in his interactions with certain characters. Also, 'What's Wrong with Secretary Kim' is a great romance manhwa novel where the boss - secretary relationship takes an unexpected romantic turn.
Arya Stark is another favorite. Her skills as an assassin and her desire for revenge can be further explored in fanfic. For example, in a manhwa fanfic, she could have adventures in new lands while still on her path of vengeance.
The main protagonists are usually popular to write about. Their complex relationship and the whole 'love is an illusion' dynamic between them offers a lot of material. For example, if there is a strong - willed male lead and a feisty female lead, their interactions can be explored in different ways.