Recommendations

Both are mini-dramas with the same male leads. One is about a new love with some fantasy elements while the other one is exploring what happens after the love ends.

They are both historical drama with beautiful cast and settings and is both entertaining and hold deep meaning for Korea history!! Both have idol actors and the acting debut of two popular kpop stars- Kim taehyung(v) from BTS and Byung baekhyun from EXO!!!! You gotta wtch these amazing drama with amazing music!!

It's about an ex-couple meeting each other again some years later,

Girl travels back in time...and influences the past with her modern views..SSL is only two EPs but still it's worth it.

Same plot...girl time travels to past. An amazing drama with amazing cast

the main couple in both dramas are really cute and have great chemistry :)

It's the second movie of this one, so it's bound to be similar.

They are both supernatural. In Gumiho, the main girl is not a human and in order to become one she needs a human. She is +500 YO.
In Goblin, Kim Shin is not human too. He is +900 YO. He wants to end his immortality. In order to that he needs a human bride.
Goblin has more interesting storyline and has a strong sweet bromance too.

Both of these dramas are about high school students have have a cutesy feel as well as supportive friends. Both have shared actors - Sakurada Dori and Shiraishi Shunya!

both have a similar feel to it. Jenius focuses more on investigation and sci-fi while Simple Man crosses many genre from sci-fi to investigation, action, mystery, medical and paranormal. Personally, I prefer Simple Man over Jenius but both are really good dramas if you are into cleverly made quality mystery, investigation type dramas.

Plots of both stories are really similar. Both are about unfortunate love in Joeson era that repeats again in our time to have happy ending. There is a little of fantasy also: mysterious time travelling in Rooftop Prince, and mermaids in Legend of the Blue Sea. Main characters have troubles adapting to modern world. And after all both series have charming love story.

Teachers with secret identities.
Yankee students.
Life lessons.

Both contain female doppelgangers belonging to opposite ends of the social pyramid, where the poor one must take the place of the rich one in a relationship with the rich lead without letting anyone know

Both involve corrupt politicians, crazy royals, and concubines. The Treacherous is a lot more gritty, but both explore themes of revenge, lust, greed, desire, and power.

Revenge plot galore, great productions, great acting (personally like Nirvana in Fire a bit more because it felt more cohesive and Hu Ge was an amazing actor... however, same can be said for luo jin in The Princess Wei Young)
The romance in nirvana in fire is more toned down than in WeiYoung. Both are highly recommended if you like smart heroes/heroines, political drama, and revenge