Recommendations

Ghosts/Spirits
ML is a Police Officer in this drama who can see ghosts
Deaths
it's a watchable drama

the plot sounds a little similar at some point
Ghosts/Spirits
Two leads in this dramas
Different ways but same purpose
Deaths back to back to cover the truth
Both have Police but in Black the Grim Reaper played the Police role as well
But still it's a watchable Drama

the plot sounds a little similar at some point
Ghosts/Spirits
Three or more Main leads
Different ways but same purpose
Deaths
Similar Exceution (Only difference is in Grid Police / Priest (Doctor,Priest,Police) were there for their exorcisms)
But still it's a watchable Drama

If you are a big fan of Agatha Christie books this kdrama was heavily influenced for the making of the revenge plot.

We only have two episodes of Twenty-Five Twenty-One but I think if you like this show then you'd really enjoy When We Were Young.

When We Were Young is a Chinese show about students in the year 1996, so there is a similar vibe going on between these two shows. Although When We Were Young doesn't have an age gap between the two leads, this show does well to really create the curiosity of "Will they get in the future?" from the viewers (like we get from 2521).

It's very wholesome and full of fun family and friendship relationships. Even though we don't know much about how the rest of 2521 will play out, I think When We Were Young is a good show to watch after/when watching 2521.

The vibe is similar. Both great general. Kinda the same story but at the same time different one. Both having a love conflict in their life. You might like.

You have the unspoken romantic vibes between the leads that a develop/have similar vibes to a slow burning romance and a show where I personally got similar vibes from (romance wise) is happiness, it’s a really cute slow burning romance, although in this serie it is spoken about even though it’s not the main theme.

They're both workplace love stories and they're both funny. In both of the shows the male leads are the bosses. They had a similar feel in terms of light heartedness.

A bit similar love affair story with different story line in Hirugao both of the leads having an affair. In Kingyo Tsuma only the FL & FL husband plus other housewives who live in the same apartment complex having an affair. If you like love affairs genre this drama is a must watch. Iwata Takanori is adorable.

A bit similar love affair story with different story line in Hirugao both of the leads having an affair. In Kingyo Tsuma only the FL & FL husband plus other housewives who live in the same apartment complex having an affairIf you like love affairs genre this drama is a must watch. Iwata Takanori is adorable.

Korean BL
Long length
Cute couple
Good looking couple
High production quality
Great chemistry and handsome actors

Korean BL
Long length
Cute couple
Good-looking couple
High production quality
Great chemistry and handsome actors

Both leads are office ladies who like to eat delicious meals after work to relieve their stress. Both dramas show the restaurant's location at the end of each episode.

Yoon Jin Ah is a woman in her mid-30s who doesn’t know yet what it’s like to date a man. She’s been dumped by a man many times because of her clumsy, reckless, and foolish behavior. And again, a man she wanted to marry dumps her for the worst reason ever: that she’s like bland, tasteless devil's-tongue jelly, which means she’s not attractive at all as a woman.
Just then, Joon Hee appears before her with a broad smile on his face. He’s as refreshing as a soft drink. Joon Hee is Jin Ah’s childhood friend's younger brother, who used to live next door. Jin Ah has always remembered him as a little kid, but one day, he comes back as a really masculine man. When she is surprised by his change, her gut feeling tells her that she would truly fall in love this time.

1. Boss- Employee Relationship
2. Assistant
3. Jealousy
4. Hate to Love
5. Smart FL
And the girl also comes here to learn something from him and both fall in love with each other at last.