I love this topic, as it always feels like we don’t get enough time with the couple once they are finally together. Special mention where there is literally a minute of them back together before the credits: Live up to your Name.
Other cute couples that would be great in the sequel: The married years or a completely different drama.

Seasons of blossom - Hamin and Somang
Love all, Play - Taejoon and Taeyang (Park twins)
Hogu’s love - Hogu and Dohee
Shooting stars - Hanbyeol and Taesung
Love in the Moonlight - Crown Prince Hyomyeong and Raon
Go Back couple - Bando and Jinjoo
Loves is for suckers - Chef John Jang and Jiwan

They are most likely dreams that will not come true still...
Kim Nam-gil and Lee Yo-won in Queen Seondeok.
Kim Gap-soo and Kim Hae-sook in Hospital Playlist.
Ju Ji-hoon and Kim Hye-soo in Hyena.
Kim Ga-eun and Tae In-ho in Kiss Sixth Sense.
Kim Ji-Won and Son Seok-ku in My Liberation Notes.
Uhm Ki-joon and Uhm Ji-Won in Little Women.

I didn't understand the question. Does this mean the actors pairing or the characters?

For characters, I want to see Seo Yul and Yoon Ha Kyung in a whole drama.

But for actors, it has been proven time and again that there is no guarantee that a pairing I have loved in one drama, I will love the same in another drama, with different story, writer, director.

I want to see Eunjoo Noona and Lawyer Minwoo from My Unfamiliar Family. I'm absolutely sure that Minwoo would love and cherish Eunjoo all his life... and Eunjoo would appreciate his talking, warmth and smiles (and who can forget Minwoo's phone call with his mom? 😁😁).

This is also a role where Kwon Yul is bright and sunny... and I want him back on my screen like that! ❤️

Be Melodramatic : Lee Eun-Jung (Jeon Yeo-Bin) and Kim Sang-Soo (Son Suk-Ku). I wish to see them in Europe and Morocco.

The King's Affection : after what they survived, I want to see them living happily or for the fun in a modern drama.

Mad Dog : Kim Min-Joon (Woo Do-Hwan) and Jang Ha-Ri (Ryu Hwa-Young). They had so much chemistry.

For me, the couple I would want to see more of is undoubtedly Iksong Ik-jun and Song-hwa (Hospital Playlist)
Those two were best friends before they became lovers and the maturity they handled their relationship, especially on Ik-jun's end was really great.

I wish we had gotten more of their dating and camping trips together. Those two were so smitten in love that they communicated without words.

I won't be able to forget this couple any soon and I wish we get a third part with them married.

I don’t understand the question either so I’m just gonna go with sequels I want:

Age of Youth 3
I really want to see my iconic Song Ji-won again okay?

Law School S2
Confirm the OTP!

Dream High 1
Reunion episode, where we see what the characters are going in their 30s

Alchemy of Souls 3
Where we see King Won having adventures with his turtle

Answer Me 1988 or 2022
Where we see what happened Jung-hwan dammit, they did him and us dirty I tell you!

Answer Me 1994
Where we see Chilbongie have his romance with Jung Yu-mi’s character

School 2015 sequel
Like @Unit said where Kim So-Hyun’s character gets with Yook Sung-Jae’s character instead

And maybe for an actual reunion pairing of actors Jang Hyuk and Jang Na-Ra, it’s about time they do another drama together again

Well technically they haven’t all been in drama or movie together but they were in a reality documentary.
So yeah a reunion between actors Choi Woo-sik, Park Seo-joon & Park Hyung- shik. Also adding Kim Da-mi to bring on the romantic tension

Before I say anything, Park Hyung Sik and Park Bo Young, hands down. We couldn't get enough of their cuteness.

A pairing between Han Hayo Joo & Yoo Yeon-seok is gonna be palpable. ( They've acted together in two movies before, but I want a full series)

Kim Go Eun & Wi Ha Joon ( mostly for compensating for the supposedly romance part of Little Women)

This ep really reminded me of Familiar Wife--the divorcing couple had a very similar to situation to the main couple's marriage in that drama.

I greatly dislike the SML. Okay, to be fair, I usually dislike SMLs, but this one is just so insidious. He's not overtly terrible, but that's what makes it worse. He emotionally blackmails and coerces the FL.

I'm also wondering now if there was more to the main leads' divorce than what we've been told.

Definitely Park Hyung-shik and Han Hyo-joo. In a real love story.

Seo In Guk + Park Bo Gum
Jun Ji Hyun + Kim Soo-Hyun
Seo In Guk + Ma Dong Seok
not the official OTP but Yoo Ah In + Park Min Young

Jang Nara and Shin Sung Rok. They were so good in The Last Empress even when Oh Sunny and Lee Hyuk's relationship was obviously not going to last for long. I want them to reunite in a romcom or any drama where they get a happy ever after~ 😀

I know I am going to be the odd one here and mention Kim Hee sun and Lee soo hyuk from Tomorrow. Please we need to be blessed with them in another drama together. All that chemistry...........it needs to be utilized.

Han hyo joo and park hyung sik from Happiness, I want more.

Second a Kim Bum X Ryu Hyeyoung reunion—WE WERE ROBBED

No brainer: 1) Shin Hye Sun and Kim Jung Hyun. Rarely have I shipped a couple so bad; and 2) Junho and Lee Se-young. Same. 💕

Ahn Hyo Seop and Lee Sung Kyung, and I'm glad they reunited in dr romantic 3. Their chemistry is just one of my fav.

Song Joong Ki and Moon Chae won as wife and husband, since for me the innocent man trajectory is such a hanging.

Ahn Hyo Seop and Ryu Hwa Young, theyjuat a side drama couple in my father is strange, but they really need to come back as drama leads i can't get enough of theem for appear only mere 5-10 min together each episode. Ryu Hwa Young is so funny.

I prefer reunions between OTPs who didn't get together (Law School) because OTPs who had a happily ever after (Strong Woman Do Bong-soon) I'd rather preserve the happy memories than watch them fall in love as different characters.

My OTPs with the best chemistry but one half died that I want to reunite
He Is Psychometric: Kim Kwon and Kim Da-som
Hotel (should have been a sageuk) del Luna: IU and Lee Do-hyun
Goblin: Kim Min-jae and Kim So-hyun

Couples I want them to stay exactly the same (aka. spin off or season 2, I don't need a new story):

—Kang Ki Young x Kim Seul Gi (Weightlifting Fairy KBJ)
—Lee Jong Suk x Han Hyo Joo (W)
—Park Hyun Sik x Han Hyo Joo (Happiness)
—Moon Chae Won x Lee Joon Ki (Flower of Evil)
—Gong Yoo x Yoon Eun Hye (Coffee Prince)
—Seo In Guk x Nam Ji Hyun (Louie and Bok Sil!!!💚)
—Park Seo Ham x Jae Chan (Semantic Error)

Couples I want to see again but in a completely different drama (aka. call the actors, not the writer):

—Moon Chae Won x Song Joong Ki (Innocent Man)
—Ji Sung x Kim Min Jung (New Heart)
—Jin Ki Joo x Kim Yong Kwan (Secret Life of My Secretary)
—Jin Ki Joo x Lee Jang Woo (Homemade Love Story)
—Choi Daniel x Hwang Jung Eum (High Trash, I mean, High Kick 2)
—Lee Je Hoon x Shin Min Ah (Tomorrow With You)

And hell yeah for Law School season 2! We need that ship to sail.

Yeo Yeon Seok & Seo Hyun Jin : I'm very curious about Kang Dong Joo and Yoon Se Jung ' s life. But sadly, they didn't join in the 3rd season of Romantic Doctor Teacher Kim.

The ones that stay with me are always the ones that didn’t quite have their romance, or happily-ever-after, and I’d love to see them in another drama where I can pretend they are reincarnations of their previous characters and actually get the happy ending this time around.

So with that in mind, I’d love to see:
Kim Hye-soo and Cho Jin-woong (Signal) (cameo from Lee Je-hoon or I will not accept it)
Lee Je-hoon and Sooyoung (Move to Heaven)
Lee Jeong-seok and Shin Hye-seon (Hymn of Death)
Jang Seung-jo and Lee Elijah (Good Detective)
Song Joong-ki and Jeon Yeo-been (Vincenzo)
Choi Sung-eun and Hwang In-youp (The Sound of Magic)
Rowoon and Kim Si-eun (Tomorrow) but also
Kim Hee-sun and Lee Soo-hyuk (Tomorrow)
Jeon Yeo-been and Nana (Glitch)

If we're talking about actors, then Jung Il-woo and Yuri. And Ji Jin-hee and Kim Hyun-joo. Though they've acted together 2 and 3 times respectively, I want moar!