With the figures done, it became obvious the background needed a lot of love to make this come together. I added texture by drawing a bunch of horizontal lines with my pixel brush and then sort of erratically erased away parts of them with my 1px eraser tool to create a random, gritty texture.
I did that a couple times with a dark and light color for both the dirt and the grass to give it depth. Then I cheated a little bit by adding a radial gradient to the base layers to create a nice, but subtle vignette to further focus the eye in the center. Then I added the chalk lines and home plate, but it didn't quite feel done yet.