Layer 8 doesn't have to be just for the sky (several Epic tilesets like Labrat and Inferno use alternatives), but it would be best if a textured background was added anyway. The more variety for the background tiles the better.

And here's an idea. Alternate versions? I think it would be great if one version had lava instead.

Edit: Fiendish Mike might have been right. Below are some different wood tiles. Compare to this.





It's unfinished as you can see.