I find when it comes to Cutie Marks, reducing noise is very important. I used to have a very detailed Cutie mark, of a sprouted leaf, on a stem, in the earth. It just overwhelmed the rest of the drawing. now I have a very simplified version. I'll include sketched of what each one looked like.
As far as Mythos Grey goes, you can cut the text off the left page, people will still know it's a book. And the dragon doesn't need so many tendrils and spikes. a small wingy lizard would get the point across, if it's breathing fire.
In StarDream's case, the problem I think you're having with regards to Cutie Mark is that that is a HUMAN eye. Doesn't look quite right on a pony's flank.
I really like Golden Dawn's character design, although I stand by what I said. Less is more on a Cutie Mark. One ghost might be a better idea than like, 5. and less stratification on the sunbeams would cut more noise from it.
All the stories you have are spot on. Yet again, I really enjoyed Golden Dawn. Reminds me of my boyfriend's OC. Anyhoo, Keep it up, and keep posting