Please keep this under your hat. The problem inherent in talking about things is that they thereby may become part of the Church's conversation within herself; and so find themselves on a possible trajectory towards realisation.
For the second time in this pontificate, there have been rumours about a Vatican III.
Perhaps this would not be a disaster. Roberto de Mattei's magisterial book about Vatican II gives an account of the coup d'Etat, the tricks and dodges by which the Rhenish bishops and their associates took the Council over; of how disastrous it was that orthodox bishops were so slow in getting themselves organised. Perhaps today's orthodox bishops would benefit from the Lessons of History. History does not always repeat itself. Particularly if people have read it.
But, given all that, three reasons do occur to me for fearing that a Vatican III may not be God's will.
(1) Vatican II had the practical effect of nullifying the previous papal Magisterium. The decisions and teaching of all the 'modern' popes (B Pius IX to Pius XII) promptly became Old Hat, superseded by the new, shiny, sexy, "documents of the Council". A Vatican III might do the same to the teachings of the post-conciliar popes (S John XXIII to Benedict XVI) and thus turn out to be a constructive relegation to obscurity of their laborious recovery of elements of orthodoxy and orthopraxy, from Humanae vitae through Veritatis splendor down to the Hermeneutic of Reform in Continuity under Benedict XVI.
(2) A Vatican III could represent a recrudescence of the horrible heresy denounced by Cardinal Ratzinger: that the pope really can do anything especially if he is acting on the mandate of an ecumenical council. It might be used by the heterodox bishops as a way of trumping all that they find unattractive in the recent Papal Magisterium. A Synod can't cancel what popes have decreed; the pope himself, if he tries to do so, puts himself in the difficult position of cutting off the bough upon which he is sitting (his own authority being, by definition, no greater than that of his predecessors). But the Pope-in-Council ... now there's the temptation ... who can set limitations on that?
(3) If Vatican I and II are anything to go by, conciliar decrees sometimes contain compromises. And, after a council, a dominant fashionable elite in the Church is left at liberty to run with its own side of the compromise and to render the other side dead in the water, a universal irrelevance. Consider Vatican II on Vernacular in the Mass. I summarise:
"(a) Latin is to be preserved.
(b) But the vernacular may be extended
(c) in the readings and directions and to some prayers and chants
(d) and to those parts which pertain to the people,
(e) but [provideatur tamen] they must still be able to say and sing the parts that pertain to them in Latin.
(f) If, in some places and circumstances, an even more radical approach [profundior aptatio] is needed, local ecclesiastical authority is to submit proposals to the Holy See" (Sacrosanctum Concilium 36, 40, and 54).
In less than a decade after the Council, (a) and (e) had become dead letters; and (b), (c), and (d) had been turned into irrelevances because the adroit use of (f), the only de facto survivor of all this legislation, had stamped the very nearly exclusive use of the Vernacular upon the whole of the Latin Church. "Some places and circumstances" [variis locis et adiunctis] had, by the touch of Circe's wand, been turned into the universal general norm. It is now not at all uncommon to find dishonest and unprincipled clerics mendaciously informing laypeople (whom, very often correctly, they clearly suspect of knowing no better) that "the Council abolished Latin". The terrible risk of similar unprincipled games accompanying or following a Vatican III should make us pause for thought. It is part of Cardinal Kasper's approach to argue explicitly that local, Particular Churches may have different disciplines. A Conciliar document might easily contain a let-out clause providing for 'exceptions' in particular places, which would within the decade become the universal norm. Our heterodox or heteropractic brethren among the clergy very sincerely mean very well, but it is dangerous to trust such people, particularly when, as sincere people often are, they are in a hurry.
More than half a century after Vatican II, we are only just beginning to transform some of Circe's pigs back into men, and finding it hard, invidious, and contentious work. Would a Vatican III do anything to wipe the sweat from our brows? Or would it simply increase the burden?