Once we place formation at the centre of ecumenical thinking, we find ourselves in a very different place from traditional Protestant full visible unity. When mission is at the centre, full visible unity makes sense. The argument goes that a united church is good for mission. Lack of unity gives the wrong impression.
Why should it? The only reason lack of unity seems negative is because we make it so. Unity expressed through church structures is no more impressive than disunity expressed through church structures. Some of the fastest growing churches are anti-ecumenical and split off from the mainstream. Sometimes this illustrates the problems we encounter when formation is not central. To go it alone is to cut your tribe off from what is common to all traditions. It isn't a sign of pure doctrine but of truncated doctrine, based on the whim of unaccountable leaders rather than on the shared faith of the people.
We need to think not upon imperial structures of top down authority but upon bricolage (the illustration is a 'Monstrous "Heath Robinson"-style contraption for cracking nuts', by velodenz). If there is trust between traditions than we can cobble together what we need from whatever is around us. It is tidy minds who create the problems; we must have a hierarchy with everything in place or failing that go it alone to preserve our precious purity.
We need in summary a culture of asking for forgiveness rather than for permission. The confidence to ask forgiveness comes through good formation, formation which makes us independent of the structures and doctrines that nurture us.