You could start out with a green diamond...applique on the outlines of the base lines in brown for the dirt, white bags on the bases and pitchers mound... Maybe applique softballs, mitts and bats in the surrounding blocks. You can find these in free coloring pages online.
You could quilt in the borders different words like: Strike one, two three.... Your outta here....foul ball....strike out...the bases are loaded, etc...