The story is about finding beauty (for which stars are a metaphor) in one's self, so that's something I'd like to see reflected in the cover. It's also about the nature of healthy relationships (think binary stars) so that's another design consideration. There is also a romance between two female characters which may or may not factor into design decisions.
A primary character in the story is named after the constellation Cassiopeia, and my initial idea was for the title design to follow the pattern of the constellation, although I'm certain the principles of designing a book cover may conflict with that.
In my conception, there are four primary "stars" in the story. The protagonist and her mother (binary), the protagonist's best friend, and the antagonist.
As far as colors, I'd like to evoke nighttime. Dark blue, white, red, and golden yellow are all strong choices.
I want something that can contrast the strength of friendship to succeed against the weight of bullying/oppression. Something that highlights the peace that exists within troubled people. Something that simultaneously evokes intimacy and pain. Of course, I don't necessarily expect all these to be present simultaneously, but I do encourage blending those and similar themes as much as possible. All that said, I'd also like things to veer on the side of minimalism.