“...that it would have been Rome that would have had to modify its position.”
Seems to me that argument is long over, P and Rome lost it. Frankly, calling the Easterners heretics because we had “cut the filioque out of the Creed” was really beyond the pale. If it is still Rome’s position, and I don’t think it is, that it can change the wording of the Creed sua sponte, then there really is absolutely no point whatsoever in any theological dialog between Rome and the East if the point of the dialog is a reunion which will never happen.
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.