It's not confirmed yet guys. Shin Se Kyung, Krystal and Gong Myung are still considering the offer. And as far as I know not even Nam Joo Hyuk is confirmed yet.
One of my favorite actresses ever. I first saw her in Healer and I was amazed by her incredible acting. She's surely one of the best Korean actresses. I can't get enough of her. I hope to see her in more dramas.
My Second Lead Syndrome List:
Hwang Sung Yeol (High School Love On)
Kong Tae Gwang (Who Are You: School 2015)
Jung Sun Woo (Monstar)
Han Shi Hoo (Orange Marmalade)
No Min Hyuk (Potato Star 2013QR3)
Lee Jae Hee (When A Man's in Love)
Shi Woo (My Lovely Girl)
Cha Eun Gyeol (To the Beautiful You)
Park Joon Ha (Innocent Man)
Goo Yong Ha (Sungkyungkwan Scandal)
Tachibana Rintaro (Last Cinderella)
Hiromitsu Kosuke (Heroine Disqualified)
But still Shin Se Kyung wasn't bad in Six Flying Dragons, so there's still hope.
EDIT: They have been confirmed. https://www.soompi.com/2017/02/22/nam-joo-hyuk-shin-se-kyung-confirmed-upcoming-tvn-fantasy-romance-drama/ I'll only watch it because there's Nam Joo Hyuk, that's for sure.
Lee Sung Kyung PLEASE
Hwang Sung Yeol (High School Love On)
Kong Tae Gwang (Who Are You: School 2015)
Jung Sun Woo (Monstar)
Han Shi Hoo (Orange Marmalade)
No Min Hyuk (Potato Star 2013QR3)
Lee Jae Hee (When A Man's in Love)
Shi Woo (My Lovely Girl)
Cha Eun Gyeol (To the Beautiful You)
Park Joon Ha (Innocent Man)
Goo Yong Ha (Sungkyungkwan Scandal)
Tachibana Rintaro (Last Cinderella)
Hiromitsu Kosuke (Heroine Disqualified)