Like maybe one of the leads finds the other leads diary and reads it and falls for them while inserting themselves into the story in some way. Or maybe they meet, fall in love in the first few episodes but then the other one passes away or whatever and the remaining lead does everything in their power to fulfill their wish. Or, one of the leads is imaginary character, at least to the rest of the world. 

Or something along those lines... 

  • With Love (1998_Jdrama)
  • MOVIES:
    • Il Mare (Korea)
    • Kimi no na wa / Your name (anime)
    • Secret / Secret That Cannot be Told (Taiwan)
    • Teacher's Diary (Thailand)
    • Kita Kita (Philippines)
    • Ditto (Korea)
    • Calling You (Japanese)


Also maybe: Someday or One Day

Not exactly what you are looking for but you might also like:

https://mydramalist.com/13724-the-secret-message - They exchange messages via Line without knowing each other’s faces (I think it was a LINE mistake or an old mobile phone number got new assigned)

https://mydramalist.com/8036-tsuki-ni-inoru-piero - a woman finds a piece of paper and start to mail with a man she never met.

its not on mydramalist but i think i've found the best one for you

The Japanese Wife.
wikipedia 

imdb

where to watch : https://www.youtube.com/watch?v=Br3K4fkt9fs 

yay! luckily its on youtube. 

this movie is from india. a love story between an indian bengali man and a japanese woman  time depicted during early 80 i guess. they start off as pen pals, then fall in love through their letters and ended up marrying over the distance 
. A marriage they maintain for almost 20 years after which they try to meet : spoiler no they do not end up meeting, the woman gets cancer, and the man tries to get to think of a cure from the city, ends up having heavy fever and dies. after he dies his aunt sends a letter to japan to his wife that he died. the japanese wife then , as she had already accepted herself as his wife, goes bald and wear white clothing ( a cultural dressing sense when one's husband dies) and finally leaves everything behind in japan and comes to india to live in his house.

Have you seen the American movie the Lake House? Or You've Got Mail? They both seem to fit this theme, to different extents. 

 kwoon:
Or, one of the leads is imaginary character, at least to the rest of the world. 
  • W - The ML is the hero of a manhwa / popular graphic novel series written by the FL's father. I'm not sure this really matches what you're looking for though, because the FL is able to enter the world of the manhwa and does meet the ML there. Later on the ML is also able to leave the manhwa and enter the real world
 kwoon:
Or something along those lines
  • Mr. Queen - Jang Bong Hwan is a modern-day chef whose soul travels back in time and takes over the body of a woman who is about to marry the king of Joseon. The King never meets Jang Bong Hwan in his own body.