Which kinds and terms of union you would *not* accept as legitimate? Which ones, if accepted by hierarchy, would lead you to not follow them into a false union?
While I trust the words of Christ that the gates of hell will not prevail...
I would reject any union that declares our accepted councils to not be Orthodox, denies the sainthood of our saints, requires us to accept councils in which we did not take part in or accept as being universally binding, requires universal submission to a single bishop having ordinary jurisdictional authority over every local diocese, acknowledges any single bishop to be unable to make a mistake when speaking/writing in an official capacity by virtue of his office, and the adoption of a form of the creed that has been historically rejected by us.
I think that just about covers all the non-Protestant traditions. As far as Protestants, they would have to accept our councils (all and whole), accept all of our teachings, adopt the sacraments as we believe them to be, adopt (if they don't already have one) a calendar including all the major feasts, a lectionary, and liturgical worship to include a divine liturgy and hours of prayer. This would probably be easier for some groups like Anglicans and Lutherans than for more non-liturgical groups like Pentecostals.
Just some thoughts.