In the beginning, ecumenical councils were generally called to confront some great crisis in the Church, usually a heresy-- Arianism, Nestorianism, Eutychianism, etc. They weren’t called just for the thrill of it.
However, at Vatican I Bl. Pius IX did invite the Orthodox to come, not merely as observers but as participants. Thus in theory the Catholics and Orthodox could get together at a Council. It is a praiseworthy thing to pray for true union of the churches, one that is based on dogmas, sound tradition and truth.