Well, here are some suggestions.
First, you may want to flip the "Metal Slug" on the spine. Generally, the titles read top to bottom. That would also mean that you would want tank pointing down as well.
While the landscape of the first level is a really cool idea, I think you may find it very limiting when trying to map additional images onto it. Of course, you would want the Metal Slug title on the cover somewhere, but as far as additional art, you wouldn't be incorporating said art into the background, so again it is a limitation instead of a help. Like I said, it's a cool idea, I think you just may have painted yourself into a corner.
The back is a great start. I would start by adding four screen shots and then either some descriptive game text (maybe something about Morden) or maybe get some "SNK Major Meg game" stuff in there just to get started and get a feel for the layout.
Hope that helps you a bit.
BTW, if you want a meg count on the bottom of teh spine, yer' gonna need to bump the other stuff north a bit.