Big age gap. FL in her 20's and ML in he's 40s.

• Marriage contract
• Sweet 18

+ 2nd Dating in the Kitchen

It is literally a gem of a drama.  Id even go so far and say its the best you can find. Though it doesn't have an arranged marriage type of a story. 

A Gentleman's Dignity (2012) - MyDramaList (one of the main couples)

Historicals

- The Story Of Ming Lan (ML makes sure to have a arranged marriage to the FL after he falls first for her and he's about 10+ older than her, she is her late teens/early 20's.)
- Shin Don (really old drama but arranged/forced by the circumtances marriage, pretty big age gap from the actors, like 20+ years.)
- Bu Bu Jingxin/Scarlet Heart 1 and 2

Modern 

- A Thousand Kisses (Secondary Couple-Fake to real relationship, Actress 20's, Actor 40's)
- Family's Honor (Secondary Couple-Actress 20's, Actors 40's)
- Golden Bride (Arranged Marriage-can't remember how old the ML is suppose to be (maybe 30's) but the FL calls him Ahjussi/Mister, she in her early 20's.)
- Daring Woman 

- Count Your Lucky Stars (20 year age gap between the actors and 10 between the characters)

Age gaps of  around 10+ years
Hotaru No Hikari 1 and 2
Dokushin Kizoku
Who Are You (2008)
I Love You (2008)
A Good Day For The Wind To Blow
The King Of Dramas
Dating Agency: Cyrano
Prime Minister and I
Mr. Back
A Beautiful Mind
Touch
Sisyphus: The Myth
Young Lady and Gentleman

 maidenholmes:

Is there a substantial age gap in Perfect and Casual (2020)? The leads are currently 32 and 25. I don't recall the ages of the characters, but it didn't seem as though he was in his 40s.

Kind of. Basically a professor and his student. But he is not old enough to be her dad. I'm not even sure if there is actually any drama set in modern China where the age gap is bigger than that drama though.

Dreams Link 

Go Go Squid! 

If you take XianXia into consideration:

Ancient love poetry

Eternal love

love between fairy & devil

Eternal love of dream

Fairyland Lovers

immortal samsara

Love and Destiny