In order for us to even convene a Synod with universal (ecumenical) authority, we have to work out the obstacles inherent in the role and scope of the papacy. This is precisely what is planned at the forthcoming Caholic-Orthodox meeting in Belgrade, Serbia.
If we can work out the role of papal primacy to everyone's satisfaction, then it is clear that the Pope can call an Ecumenical Council to which both sides of the Church will respond. As long as we understand that our theological differences may not be true theological errors but theologumena within the bounds of Tradition, we can look forward to a true, spiritual re-union, and communion.
The same cannot be said of the Protestants because they are not in the Church. Their teaching is heresy, by definition (i.e. outside of the teaching of the Church). We cannot accept their teaching as aberrant opinions because they have forsaken the authority of apostolic succession and, by that act, the validity of their "clergy." They are apostates, even though they profess faith in Christ (so do the Mormons, and JWs, and even the Gnostics!).
Within the Protestant amalgam there are groups that are close to the Church, such as some Anglicans and some Lutherans, whose return to the Church is possible (and in the case of Anglicans eer more so likely). But those who deny Trinity, consider Christ a brother of Lucifer, a lesser God, or who profess double predestination, that God created evil, those who ordain women as priests and bishops, those who profess that the Eucharist is only a symbolic presence of Chirst, that Chirst has only one Nature, etc. -- no matter how devout they are in their beliefs, and I believe they are -- they can never be reconciled in the Church because those teachings are outside of anything knonw to the Tradition.