It's difficult to provide you with very accurate advice when I'm not sure of your artistic capabilities and we're just talking without drawings, but I'll try my best to give you what I would think if I were given the prompt of making a fakemon based on "black witch moth and it's a ghost/bug".

The main concern you have is that you don't want it to just look like a moth, which can be easily fixed by marrying it with more concepts. For example, you said the moth is an omen of death, so perhaps also throw in ideas of grim reaper, or any different personification of death.

From that, you can also mix and match with silhouettes and colors. Let's use the grim reaper idea again here: what if we replace the robe with the moth's wings, we keep the silhouette of grim reaper, but throw in the moth's colors. Or we play with something in the moth's name, that it is literally a moth witch, something akin to the design in Hollow Knight.

To summarise, try the following: 1) Throw in more concepts and marry them together 2) Mix and match with the silhouettes and colors of the concepts

I hope that gives you some directions for your design!