I've been reading webtoons (and manhwas) for years now and some of them have already been adapted into kdramas. There are some adaptations that I loved such as tale lf nokdu and cheese in the trap (except for the ending. God, I hated the kdrama ending) and some that got me disappointed by (bride of habaek). Anyway, I think there's a lot of webtoons that really deserve a kdrama adaptation.
I'll begin with Medical Return.
Synopsis: Kim Jihyun, who lived his life as a disreputable surgeon, gains a second chance to relive his life. He then goes back to his middle school days, and live his life to the fullest unlike before, he studies nonstop. His goal is to become a dermatologist who make lots of money instead of the poor surgeon he was from previous life!