Maybe a little fisheye distortion on the background? It looks like the background is a kind of a planet shot from space. The curve is down like you were looking at a sphere instead of up like you were looking at the inside of a sphere.

You should also consider the total layout of the image. A paperback or dust jacket wrap around the book. Usually the publisher wants a prominent title on the front cover and spine. The back cover often will have a teaser paragraph or text such as "second in so-and-so's epic adventure of bla-bla-bla. Many artists center what would be the normal focal point of the image within the 2/3rds of the front cover. Places where the publisher will slap text often are painted with a background. In your case, a continuation of the hollow world. This off center style of galley proof is the prefered layout for covers.