By "sua sponte" do you mean without the East, or without a future Council? Seems to me the same authority that the Fathers of Constantinople in 381 had to elaborate on the original Nicene Creed of 325 must also extend to future Councils as well, or else we are denying the power of Councils altogether.
I don't think there is any precedent for making one particular Council a sort of super-Council whose canons override all the rest. That's what our folks are trying to do with Vatican II, and it ain't pretty! :)
And of course the "cutting the filioque out" argument is now shown to be ridiculous, historically, but I wonder if it didn't seem that way to the West because we had always implicitly understood the filioque in the Creed to begin with--even when it wasn't explicitly stated.
“By “sua sponte” do you mean without the East, or without a future Council?”
Without a future ecumenical council which necessarily means with the East since by definition the council must be of the Oecoumene, the commonwealth of God’s people on earth, The Whole Church. But even then, all such a council could do would be to elaborate, not change. Could “filioque” be an elaboration? If one is honest, of course not. To accept that the filioque means something other than what it clearly says is disingenuous, for example, to say that it means “from the Father through the Son”, which is of course true. It is also true that filioque doesn’t say that. Filioque, per se, denies the monacrhy of the Father and that’s heresy. Now we can all pretend that filioque in Latin and English or French means “...”from the Father through the Son”, but doesn’t that just bring contempt on The Church? Rome was wrong in the filioque formulation. It should simply admit it and scrap the fig leaf.
“we had always implicitly understood the filioque in the Creed to begin with—even when it wasn’t explicitly stated.”
I don’t believe that for a minute. Neither did centuries of popes, which is a good thing because, as I said, if filioque means what it actually says, as opposed to what one might wish it says, its a Trinitarian heresy.