Both of these dramas feature a character getting stuck in a fantasy world or a fantasy character getting stuck in the real world.
But have romance, both have cute side characters, and both have open endings.
Pop Out Boy is a light, cute, webdrama while Memories of the Alhambra is a bit sadder and was aired on TV.

Both Pop Up Boy and Lost Romance deal with falling in or out of a fantasy world.
In the case of Pop Out Boy, the ML is thrown into the real world. He used to be the MC of a comic book.
In Lost Romance, the FL is thrown into a novel.
In both of these, one character loves the other and is constantly rejected. However, they eventually melt the hearts of their partners. Both of these dramas also use the plot of "running out of time" in their new world. They also use plenty of comedy which is always fun to watch!

These dramas both feature a couple going back in time to the prime of their youth after a tumultuous life in their late twenties.

Both of these dramas feature unrequited love by the FL where the ML has no feelings for her. Both of these have really crazy plots (in a good way).
Both have the ML having some sort of trouble with other females, and both have SML's who are really nice.

Both of these dramas are about a rich (and dysfunctional) family who is torn apart by the sudden death of one of the members. The family members begin to weave a web of deceit as they try to cover up their crimes while also trying to incriminate each other.

These two dramas are both about a man (the one in Prom Pissawat is half Korean and in Love Beyond he's half Chinese) travelling to thailand to find their mother.
While Prom is way more angsty than the childishly comedic Love Beyond drama, they both feature many side characters as well as dramatic reveals of who the men's real mothers are.

Lost Romance is about a girl going into a romance novel and meeting someone who looks exactly like her crush. In real life, her crush is in a coma, but in the novel he slowly develops a romance with her. Of course, everything that happens in the novel can't affect real life... or can it?
Someday or One Day is about a girl travelling into the past and meeting a boy who looks exactly like her dead boyfriend... except he isn't her boyfriend... or is he?
Lost Romance is really funny, meanwhile Someday or One Day will leave you with tears trailing down your face.
Both of these dramas have the same actor. The second-male-lead in Lost Romance makes a cameo appearance in Someday or One Day (hint, if you want to see him kiss a boy...)

To All the Guys is a reincarnation story between two lovers.
Someday or One Day is a time-loop story about two lovers travelling back and forth in time to meet each other.
To All the Guys is suspenseful and comedic while Someday or One Day is suspenseful and tragic.
If you liked the inclusion of LGBTQ+ in either one of these, watch the other one.
To All the Guys had a gay misunderstanding (which many people did not find funny), but it lead to the FL expressing her acceptance of the community.
Meanwhile, Someday or One Day had a small sub-plot where a boy was bullied to death for being gay.

Both of these drama have amazing production. They both focus on past lives and present lives. However, Extraordinary you is about the present and past lives of comic characters. Meanwhile, Someday or One Day is about a girl and boy travelling back and forth in time to meet each other.
Extraordinary you and Someday or One Day both have similar endings and they have just the same amount of amazing plot twists and moments that make you want to cry.
The amount of second lead syndrome in both of these dramas is overwhelming, but you'll find yourself rooting for the main leads in the end.
Extraordinary if both funny and tragic at the same time. It goes over the issues of having a heart disease and unloving families.
Someday or One Day goes over letting go of the deceased, being depressed, being gay, and being born with impaired hearing.

W is based on the concept of travelling back and forth in a comic because of a loved one.
Meanwhile, Someday or One Day is based on the concept of travelling into the past, present, and future because of a loved one.
Both have bittersweet endings, and both have amazing production.
While W is pretty comedic, Someday or One Day will have tears rolling down your cheeks.
Both of these dramas also have amazing plot twists that just leave you stunned (SPOILER: especially when it comes to people possessing each others bodies, you'll get what I mean if you watch either one of them *wink*)

Both of these dramas also use the plot device of reincarnation.
Both of these dramas have the same FL. And both have very good bromance.
Both of these are also very comedic while being mysterious at the same time.

Although these dramas do not have similar premises, they do have one similar plot device. Both of the couples in these dramas knew each other in their past lives, and the FL's both died because of the ML's.