I think my next step for mapping United States Unitarian Universalist congregations is to assign each congregation to a commuting zone. Three years ago, I identified Unitarian Universalist congregations by micropolitan area -- where appropriate. But commuting zones cover the whole United States, and since they "are geographic units of analysis intended to more closely reflect the local economy where people live and work" (source, USDA Economic Research Service), they are more likely the show the organic relationships congregations do or could make. (Outside of eastern Massachusetts anyway, given its particular history. And how traces of seventeenth century boundaries survive.)
Might also show what's the "natural" core of a rural outreach; say, one that's legitimately too small, remote or both to expect a congregation to spontaneously gather.
It may be worth analyzing some of the assumptions I made. I don't like them all, but I think this is where we Unitarian Universalists are.