There are some age gap romances to be found in c drama.
The sister of the ML in Loving, Never Forgetting gets gradually romantically involved with the older male secretary of the family. There's a huge gap between their ages, but also in status. I enjoyed their story much more than the romance of the leading couple.
The age gap between the FL/palace girl and the ML/emperor in Story of Yanxi Palace is more than 16 years, their love takes ages to develop long after her romance with the SML. I loved their chemistry and funny quarreling scenes.
In Eternal Love is a side story about the love between Dong Hua Di Jun, the old Emperor of Heaven, and Bai Feng Jiu, the teenage fox goddess. Their story has its own spinoff series, Three Lives, Three Worlds, The Pillow Book Recommended!
