I've always imagined a dragon being the central theme of this cover, perhaps flying in the foreground toward the viewer, or otherwise very central. The backdrop should reflect the world - it's frozen, mountainous, glacial, etc. Lots of snow, rock, and so on. I've got plenty of artwork which gives a good idea of the environment and color palettes.
One important aspect is that the world is very dark and 'gritty' in a way - there's vast slums, dangerous winter storms, roaming bandits, and almost everyone is a thief in one way or another. It's less 'fantastical' or magical, and more serious and dark (but it's still not grimdark or anything, just quite a rough place to live).
The dragons themselves are a little different to normal - They're serpentine and have four legs/two wings, and their colors are whites and blues. I'd prefer a longer, tapering head/snout to fit the serpentine feel. In the book they're always described as having a big 'mane' of blue-black spines, kind of like a lion's but extending down the shoulder-chest area. It's sometimes referenced as being like a porcupine.
Because this is YA, I also feel it's important to have the main character (Ava) on the cover as well. These covers give an idea of what this might look like:
Because this is YA, the character fits in the sort of 15-17 age range. She's always described as having blue eyes and long white hair.
For typography and font, I'd love a sort of medieval-looking font, with serifs and sharp edges. Here are some examples of the fonts I'm looking for:
For cover dimensions, when I get a print version done (which I'll be looking to work with the winner later on to do), the dimensions will be 5.06" x 7.81", so a little more 'square' than the usual 5 by 8.
I'm really looking forward to seeing what you can come up with!