I've been designing fantasy art for myself recently, including mermaids of all types for a line I plan to market myself. I'm well versed in mythology, fairy tales, legends, and comic book history, all of which are magical, fantasy beings and themes. I'd like to propose an image of a woman's face in partial profile, with bright white light illuminating part of it with a glow. When I think of ethereal, I see angels, pure maidens, light beside clouds, lily flowers, crystal clear water with light reflecting from it-perhaps coming up from mists. These are just starter ideas I'd like to suggest however it would help very much to know who your target audience is and what the product is used for in case it could also be represented in a symbolic form. Knowing an audience whether international or local, gender specific helps further define the design ideas I'd offer.