Well, when i need help making an artpiece I tend to use music or poems to help with the design process, you could take inspiration from "The Raven" - By Edgar Allen Poe. It's one of my personal favorites, its so interesting and I highly recommend it ^^. You could also take inspiration from album covers. since you're going with a "sad" type of theme you could take inspiration from Billie Eilish's "Hit Me Hard and Soft" album cover. Or "When We All Fall Alseep" album cover. these are just some ideas, good luck ^^