Recommendations

Both best friends falling in love, and missing opportunities to confess

Somewhat of similar situations. Family issues...Finding love... Female main lead both have strong and independent characteristics. I get the same feels when watching the dramas.

The two stories take place in the same historical time. Faith takes place in the last part of Empress Ki, after she gets married.
While in Goryeo reigned King Gongmin and Queen Noguk (Faith), in Yuan did Emperor Huizong and Empress Ki (Empress Ki).
The two states were in constant conflict with each other. In Faith, King Gongmin had to fight against those who wanted Goryeo to be another province of Yuan. And one of those, was the main antagonist, Ki Cheol, brother of Empress Ki.
Both are historical, action and romance dramas. Only that Faith it's a lot lighter, have fantasy and medical themes, and focused in a romance story between two characters. Empress Ki is a darker and more serious story, with lots of tragedies and political themes, that focuses on a longer range of time, and in which the romance is just another one of the many points that has to offered.

Both are super sweet romantic comedies with a touch of melancholy, dealing with different kind of sickness. They give a similar warm feeling.

Both having to do with the main male lead having to complete some sort of mission undercover with someone at the side who helps. Also the main male leads has to protect someone in order to complete this sort of mission.

The main female lead is a fan/in love with the second main male lead but ends up falling for the main male lead and is also main reason why bad people are going after her.

The second main male lead seems to have an optimistic personality on the outside but on the inside he is torn apart.

Both have a lot of action.

Both are based on well-known manga and give an insider look at the lives of mangaka (the creators) and the industry (in both cases, some form of Shonen Jump).

In both series the men find out after a few years that they have a child, the family doesn't like the mother and try to steal the baby. Both show the parents having other partners, both take a feminist path.

Actually it is not a historical period drama like this one yet you can watch it if you hate the end like me. Indeed, Scarlet Heart 2 is the second part of Scarlet Heart chinese version. In this version , you'll see the two main leads in the 21st century.

Both are stories about medicine and particurlarly surgery. Despite the difference about the summary , you surely will like it too! Both dramas are played with great and charismatics actors!

Both of the shows are in office settings. The female leads in both shows are strong and very funny. There's also a love triangle between the girl, her ex, and her coworker.

Both of the shows are in office settings. They both have angry bosses.

About the life of former idol girls..how they fare after they're not in the spotlight anymore.

Both are about daughter-in-laws that can't seem to cope with the strict life of their husbands and have conflicts with their new family members.

Its because the both shows depicts a determined girl who loves a very popular guy and try to Achieve his heart and love. Here also we could see a love triangle .... Try it.
Note this drama is based on business rather than highschool and college

Both dramas have a similar plot line about three men with different personality handling a baby.