The yeast metaphor is one to think about, but mostly, this bread is not rising.
I hope people will come to realize that progressives are really very ordinary people. These folks I know in Birmingham are about as unthreatening as a bunch can be, and they all live within existing denominations. They are not the "other" in the same way that UUA and UCC seem to be.
Out here in the wrong-colored states it is a given that no democratic candidate from New England is going to do well in large parts of the country, especially someone from Mass. There is already a prejudice in place in the minds of a lot of people. It involves those two denominations with all other things from the northeast, the home of the dreaded Liberal. Unfortunately those of us in other places have to get around that prejudice by coming together more visibly and carving out more of a public identity.
I understand that another denomination might not work in Texas. It might not work in Atlanta, either. An idea then is to become involved in whatever progressive organizations are within your reach. Some of us in the wrong-colored states end up hanging with the Unitarians, even if it is not a good fit, for lack of a better one among the Christian churches. I told my UU minister once that if there were a non-trinitarian Christian denomination, like the Unitarians and the Universalists both used to be, that I would join it. There is not one; and it is like not being able to buy shoes that fit.
So, folks, if not a new denomination, what is Plan B?
They are spread thin. The question is whether they, or any of us, can be more effective scattered widely among the other churches or in a concentrated group.