Personally I don't see outward visible unity as a realistic possibility this side of heaven, at least not legitimate unity. Christians, by definition, enjoy a basic unity in their common faith in the Trinity, the deity of Christ, salvation by grace through faith, etc. We generally understand what divides Christian from cult. Jehovah's Witnesses and Mormons are not true Christian churches by the historic understanding of that term. Neither of these organization accept the fundamental Christian understanding of God, salvation, the person of Christ, and a whole host of other essential creedal beliefs.
Unfortunately as the older mainline denominations age this traditional understanding of the fundamental essentials of the faith appears to be eroding. A willingness to grant equal footing to other faiths as legitimate expressions of Truth causes denominations with historic Christian identities to shed the exclusive claims of Christ himself. Thus, Jesus cannot be THE Way and THE Truth and THE Life if all roads ultimately lead to heaven. Expressions of God have also warping into unrecognizable shapes. Take the example of "herchurch," also known as the ELCA congregation Ebenezer Lutheran Church in San Francisco. They no longer worship the revealed God of Holy Scripture, but now recognize the deity as "Goddess." So even paganism is now placed alongside of Christianity with equal voice. Yet that never works, for one voice always dominates. In the case of herchurch the voice is that of paganism. In the description of their proposed "Goddess Mural" write: "The Goddess mural will rise 64 feet high with four basic symbolic representations of the divine feminine: The Earth Mother, the Black Madonna, the Christ-Sophia, and an androgynous Kali-Kundalini figure whose chakras merge with the phases of the moon and the universe." The last figure is clearly a Hindu deity. The "Earth Mother" reaches back into primitive mythologies and other non-Christian religions.
So given such a diversity, where do we start? Should we even attempt such a thing? Would we not surrender the Faith itself in the end by compromise?