I'll keep this short. The conventional wisdom says the United Church of Christ and the EpiscopalÂ ChurchÂ -- sometimes the Lutherans and the Disciples -- have filled the ecological niche of liberal Protestantism that once included Unitarians and Universalists, thus we don't need to form new Christian churches, or perhaps only benefit from planting those that speak from a very tight theological perspective.
Missological concerns aside, the original premise isn't true. Denominations have regional clusters and variations. I think Unitarian and Universalist Christian churches have room in empty ecological niches, say where the Congregationalists did not go, and where the Episcopalians skew conservative.
The upcountry of South Carolina comes to mind; also central Virginia? Â Western and southern Illinois? Pittsburgh?
Besides that I'd scan the late-to-ordain-women Episcopalian diocese and areas with strong UCC attachment to the ostentatiously-named Biblical Witness Fellowship.