Love at First Taste the boyfriend is an alien while in My Girlfriend is an alien the FML is the alien.
They both fall in love and both have something in each others body that the other needs to either be alive of go home.

Angel's Last Mission and Love at First Taste are both similar in the points of both FML meet someone that isn't human and fall in love with them.
While one is just the career is down the drain the other will die soon.

Gods, romance, and guardian angels all similar points but two different plots. The Devil Punisher is more about the romance between two beings that work over earth while Rainless is about a human and a Kawas.

Similarities:
-Princes falling in love with a maid
-Rebellion
-inter competition between all the princes
-lover triangle
-The characters Wang Wook (Moon Lovers) and Yan Xun (Princess Agents) have a very similar goal, but end up hurting the one they love

Princess Agent has defiantly more danger and action pact.
-secret organizations

Moon Lovers: Scarlet Heart Ryeo focuses more on the romance and relationships between the Female main lead and the Princes.

Both The Player and Love O2O are similar based on online gaming.

While Love O2O focuses more on the romance between the two character inside the game and outside. The Player focuses more on the interaction the gaming world is to reality.

Both The Player and The King's Avatar are based on an online gaming competition with individuals working.

While these two are very different there are some similarities such as
-Both are related to planes
-Both have teamwork

One the other hand:
The Eyas is more about the training of young adults to become pilots in the Airforce.
The Captain is about the crisis that happens mid-flight.

The only similarities are to do with time. Sisyphus is more about going back in the past, while TKEM deals with going back and forth between two different worlds.

The only reason I am recommending these two together is because of the Yo-Yo kid in The King: Eternal Monarch and So Nyeo Shin (The Deity) in Doom at Your Service.
They have similar roles as the ones not being a main character but having a huge role in the story. They are the ones that watches over everything and only interferes when they deem necessary.

While The Eyas is based on the Air Force, KungFu Cop is based on S.W.A.T. but eventually the same concept. They bring together the best of the best to train them.

Both are about learning about how to fly. though The Eyas is more focused on the air Force and there isn't a lot of romance in The Eyas they focus more on the friendship they create while training.

In Revive the ML dies and gets reincarnated in another young soul. He goes on to finding out who killed him and go back into acting. More bromance
In Two Souls in One the ML gets into a coma and his soul transfers into his sisters body. mostly romance
They both have actors that go back and start from the beginning of their careers.