I also spent some time today re-designing the ground tiles for most levels. I will simply re-color them for other levels, as well as design a few more variations for more unique layouts, but this is the basics of what I have so far:
You can see that there's a lot more detail added in, while still keeping the theme of squares and rectangles. I discovered that blending two colors with GIMP's smudge tool, using a noise filter and then using the Posterize effect as the last step really helps to make graphics look retro, and it's easy enough to pull off! These really look somewhere between 16 and 32-bit graphics, eh?
The shadow underneath the grass sections really helps to make them "pop" out more, and the squares in the dirt actually serve two purposes.
1) - To reference Sonic the Hedgehog (one of my favorite games of all time!), and
2) - To hide some sections that I couldn't quite make seamless. The squares draw your eyes away from the inaccuracies, and literally cover some of them up! If you look REALLY closely you can still some stuff, but otherwise you wouldn't know they were there!
Now I just have to re-design some of the other sprites, and I'll be good to go!