I like to give you guys a lot of latitude - in my previous competition it was very interesting to see the diverse entries. That being said, big bonus points if you can somehow incorporate a map/design/general outline of Dante's "Inferno" (may or may not be possible due to copyright issues).
If you'd like to make the cover absolutely insane and over-the-top, that'd be a good way to go, it'd certainly fit in with the story. War, explosions, crazy symbols, etc. The main villain, Cain, noticeably has a pair of golden/yellow eyes (that I totally didn't steal from Naruto). If those could be incorporated into the cover in a cool way, that'd be excellent. But if it looks dumb, then don't do it.
[Dimensions: Cover should be 7 inches x 10.25 inches, spine should be 1.86 inches x 10.25 inches]