Recommendations

Timeline Changes but only 1 year back
Thriller/Mystery
Serial Killer on the loose
it will give you the vibes of what will happen next

Timeline Changes but only 1 month
Thriller/Mystery - check
it will give you the vibes of what will happen next if the timeline changes

Timeline Changes - check
Thriller/Mystery - check
it will give you the vibes of what will happen next if the timeline changes

Both movies are romantic comedies with a focus on sensual/sexual topics that are most often not openly taked about. "Love and Leashes" deals with BDSM and the romance plays a subordinate role (pun intended), while "My P.S. Partner" is all about phone sex, satisfying a persons needs and cheating in a romantic relationship. Both are quite funny and surprisingly sexy.

Both movies delve into the world of BDSM and how it may free the people involved. "Love and leashes" gives more of an introduction into the matter for someone with no prior knowledge but "Nana and Kaoru" as well shows one experienced partner and one newbie who discovers this new world. Both movies are ultimately comedies and have a slither of romance in them even though that is not the main focus.

Koishikawa Miki, a high school student, is told by her parents that they are getting a divorce and swapping partners with another couple. The other couple have a son called Yu, whom Miki becomes attracted to after moving in together.

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

Same ML actor, director and writer and also a fantasy seeting.
Both FL are fashion designers.
Both working with a ghost theme, even in "Look Mai Laai Sonthaya" it isn't a real ghost.
In "Look Mai Laai Sonthaya" the ML have superpowers.
Give it a try.

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.