I think people would be right in the Orthodox world in this day and age to worry about a modernizing craze arising and justifying itself by a council called with a vague agenda or such a council’s supposed “spirit”, a la Vatican II. I am of the old school who believes councils should only be called to address particular urgent heresies. I hasten to add that I do not object to Vatican II’s magisterial documents, which have their good points (and I am not a novus ordo hater, so long as the reformed liturgy is done reverently and with better and more traditional music).
The original tentative agenda, drawn up in the 70s was a modernizer nightmare. So far as I know, its been scrapped but truth be told, I don’t trust our crowned heads as a group any more than I trust yours as a group. So the longer the “planning” goes on, the better as far as I am concerned. Like you said, Great Councils should be called to address particular heresies or maybe even a Great Reunion, but not, as one of the 70s suggestions had it, our fasting rules and liturgical forms (sound familiar?)!