Both this drama and the film Tenshi no Koi deal with an age gap relationship wherein the male is the older lead.

Both deal with disease and illness in characters and both seem to have underlying deeper meanings.

Both delve into themes of death and similar themes of reincarnation
They both deal with an Ahjussi romance
Overall, similar fantasy-romance themes
Similar supporting character roles - Goblin's Yoo Deok-Hwa VS Mr. Baek's Choi Dae-Han