Unitarian-Universalism was born from the combination of two historic traditions. Together they forged something new and unprecedented: other church mergers were re-unions of previously divided denominations (Northern and Southern Presbyterians, etc), but the marriage of Channing and Murray's churches was the making of two disparate bodies into one flesh. The result has been a one-of-a-kind denomination, pushing the boundaries of American religion and regularly redefining the possibilities of such hoary concepts as church, faith, tradition, and even religion.
What is needed at this moment in history is a new merger. Best candidate: the United Church of Christ. We are already close kin, all descendants of the original American religious experiment and leaders of the liberal tradition. We share similar polity, church culture, religious orientations, social concerns, ordination requirements, and even geographic distribution. Our combined strengths would spawn a new chapter in American religion that could revitalize the ailing religious left.
Runner-up alternative partners: Society of Friends, Reform Judaism, Community of Christ, [Your choice here]