Faith
    This would be an idea for churches wanting to have a landscape that doubles as an educational Biblical garden that includes plants that have religious significance whether originating from a direct reference, a tradition, a legend, or symbolic with respective examples of plants being pomegranate, Judas tree, dogwood, and passionflower.  Where necessary/possible substitutions with native species will be used and in some cases this is even due to translation errors such as for sycamore and weeping willow, which is actually a sterile hybrid.  Some additional details are discussed under the topic of water features, trails and paths.  One rule is that each bed have just one kind of plant, see Leviticus 19:19 for reference.

Color
    One of the easiest and most popular color themes is white due to this being such a common selections due to the numerous ways a mutation can effect pigments where in many cases the result is none thus flowers are often white or may lack specific colors, or example rhododendrons may lose pink but still have dark spots and even yellow.  A recent reportedly sterile variety of rose of Sharon has totally white flowers because even the red deep inside the blossom is missing.

Other
    There are many more possible themes than can be covered here, but a few of these include edible, medicinal, and/or historical where one of my personal favorites is plants discovered by William Bartram and/or described in his journal Bartram's Travels written while he was searching for plants in the southeast during the 1770s.