A mystical person appears in order to bring out the dark side of human beings. The motivation is not totally clear, but both main characters seem to enjoy the suffering they create.

Totally different story but similar vibes including entangled fates and characters which share a past together. Both are morally ambiguous and have a dark setting and still it is possible to emphasize with the characters

FL enters an ancient time world and suddenly is the wife of the crown prince/emperor. Her husband mistrust her. Is somehow mean but also interested in her.
The FL is strong minded, selfconfident and funny. In both she try to find a way back home but the focus is on the hate to love relationship.

The FL approaches ML because she know a beloved one was harmed by him and his family. Step by step she finds out what happend and struggle because of her twisted feelings.

The ML fall for the vengeful FL at first sign before she starts to avenge. He belongs to the family she wants to harm, so he tries to protect his family and struggle with his feelings for the FL.

Quite similar romance setting between the leads - one likes the other one-sided first while the other did not think about dating a guy before (If this contains to much spoiler, please let me know)

Both are parodies about the world of Asian drama / manga in which a person in this world experience it as an outsider. While Zettai BL ni Naru Sekai vs Zettai BL ni Naritakunai Otoko deals with BL Manga, Drama world deals with Korean Drama. They use a lot of clichés a fan will recognize instantly

A child enters a very dark gangster world in a very young age. In both dramas they do horrifying things for a parent-like person. Some day they get confronted with the normal world and kindness - they start to struggle who they want to be.
Both dramas are pretty dark. The gangsters also have some persons they care for,

A girl dies and meet the grim reaper who tell her that she has 49 day to say goodbye and resolve things she regrets

49 is more about solving the case of her death while Itsuka, Nemuri Ni Tsuku Hi  is more or less only about letting go and leave without regrets (which is also a part of 49 days).
The relationship of FL and grim reaper is quite similar.

A soul is caught on earth and cannot leave. This soul meets reincarnated souls from his past life and together they figure out what happend to them.

A woman has to deceive man. In the beginning she is controlled by someone else. However, at a certain point she will follow her own agenda and develope feelings for her target.

Both dramas deals with shaming and plotting of rich family members against each others and a ordinary woman who gets drawn into this world.

ML misunderstand FL and thinks she is a person without honer and pride. The FL who is a good person defend herself and try to avoid the ML. However, by chasing after her the ML learn about her true colors.