Alex

Gender swapped in a way. Character's father was murdered by a thought to be friend, who then marries his widow. Child grows up in a family that mistreats them due to being unrelated.
Main characters briefly meet while children, then reunite in adulthood.

Gangster world/setting.
The subplot of Dae Su & Jung Ae in AI reminded me of Gil Ho & Jae Hui in GBS.

Both legal dramas.
Both feature a mentor/mentee dynamic between female lead and an older (middle aged) female boss.
The middle aged woman goes through a divorce and has a child with ex.
Female centric, slight romance.

Criminal father with an evil son and sweet but naive daughter fall victim to a revenge plot from the son of a single father.
This son seduces the young daughter of the family as part of his plan.
Hit and runs also important plot points from both.

Genius and eccentric university students assist police with cases.
Female leads have a crush on the male leads.

Female lead becomes a cop/detective after the murder of her sister to seek justice.
Forms a new sister like bond with second female lead.
Older male cop/detective helps female lead become one herself and serves as a bit of a mentor.
[ SPOILER ] Male lead female lead grows close to while working later dies.

Romance genre with hard hitting/insightful life circumstances.
Female lead is raped.
Male lead doesn't know it happened for the first few episodes, later finds out. Helps her overcome it.

Enemies to lovers.
Leads start out proper enemies- bloody fights and torture.
Female lead somewhat damsel in distress like while still strong and badass.
Male lead's cold attitude turns through gradual love and admiration for female lead.

Ensembles.
First half focuses on the leads in their teen years.
Second half focuses on them further into their adult lives.
In the first half, the male lead likes the female lead's best friend. In the second half, he comes to realize his feelings/fall for the female lead.
Male lead's best friend also has feelings for female lead.
The second female lead disappears at the end of the first half.

Leads knew eachother in high school, reunite as adults.
Male lead pursues female lead after reunion.
Male lead is sick, female lead falls for him without knowing he's dying.

Letters travel through time through a specific mail box.
In TL, the leads are both in the past and already know eachother. In IM, the leads are split, one in the past one in the future, and they contact eachother through the letters only.

Male lead aims to get revenge on female lead's father for incidents that took place while he was a child.
Female lead is a quirky and spoiled rich girl. She quickly falls for male lead.
Male lead painfully falls for female lead despite revenge being his priority.
Female lead's father is the owner of a hotel in both.