Another theme is exploring the backstories of the characters more deeply. In the actual drama, there may not have been enough time to fully develop all the characters' pasts. Fanfictions can fill in those gaps, like delving into the character's family history or how they came to be the person they are in the drama.
Another place could be Wattpad. There are many Kdrama fans who post their fanfictions there. Look for the Snowdrop section or do a general search. However, the quality may vary, so you might need to sift through a bit to find the really good ones.
Once upon a time, a fisherman in a Chinese village found a mysterious box in his net. It could be filled with precious jewels which would make him rich (good). But it also might contain something dangerous or bring him bad luck according to the superstitious beliefs in the village. This is a typical'maybe good maybe bad' Chinese story.
One great Kdrama to watch for a love story is 'Crash Landing on You'. It tells the story of a South Korean heiress who accidentally paraglides into North Korea and falls in love with a North Korean army officer. The chemistry between the lead characters is amazing, and the show beautifully depicts their journey of love across different cultures and political boundaries.
One common theme is unrequited love. For example, in many Kdramas, one character loves another deeply but that love is not returned immediately or at all. Another theme is separation due to external factors like family differences or social status. Just like in some dramas where a rich heir can't be with a poor girl because of family opposition.
You can start with an uncertain event. For example, 'One day, I received a mysterious letter. I had no idea who it was from or what it contained. It could be good news or bad news.'
A boy bought a lottery ticket. It was a'maybe good maybe bad story'. He was excited about the possibility of winning a large amount of money. When the results came out, he didn't win the big prize. He was really disappointed at first. But then he realized that he had spent very little money on the ticket. And this experience made him understand that he should not rely too much on such uncertain things. Instead, he should work hard to earn money. So it was a bad thing in terms of not winning, but a good thing in terms of the lesson he learned.