A few ideas I have are as follows (but by all means feel free to run with your own as long as it meets the requirements I listed at the end of the Content Details):
*There's a scene in the book where they're fighting armored Knights in a forest that might serve for a good cover. The enemy knights are all wearing livery over their armor portraying a Feudal Japan-style crest (look up Feudal Japanese crests to get an idea for this style) that is a red bear's paw, claws included. The main character wears dark maroon PacRim-style armor as well as an Iron-Man-style helm. His house crest is a white tulip. The enemies' armor is of varying colors, but they should all wear the same liveried vests. They generally fight on foot, men holding large shields in the front rank while men in the second load and fire single-shot plasma rifles or wield long pole axes. The bandits wear mismatching armor, some of it the fancy style, some of it normal plate, some of it chain mail, some of it padded gambesons or even hardened leather. They also often wear green or brown cloaks for camouflage, and a few should bear their own bandit crest which is three trees with their branches and roots entangled with one another. They also have some single shot plasma muskets, but most bear either spear-like weapon, crossbow, or longbow.
*It might be cool to have the cover just be the main character mounted on a horse, his helm's faceplate raised so that we can see his face (he is black and looks like a young clean-shaven Idris Elba). In the distance on a hill behind him is his family's estate, which has a large stone castle in the center of thatched-roof cottages and larger tiled-roof houses. He should look determined and eager for a fight. If you opt for a close-up, his helm should be obviously future tech so that we have both the Medieval and scifi clearly visible. If you go for a wider angle to show his horse, make sure his weapons can be seen - he carries a long-hafted flanged mace as his primary weapon, wears a dagger on his right hip and a couple of single shot plasma pistols on his left hip. He can be raising his mace in the air or something, perhaps even giving a battle cry and maybe with banners in the foreground for his bandit army. Again, the key is to show both the scifi and Medieval motifs.
Honestly, I just want a great cover, so if you come up with a better idea, please run with it.