I don’t think it plausible that Benedict XVI wanted to carry on being the successor of St Peter and not bishop of Rome in order to allow someone else to be bishop of Rome and not successor of St Peter, i.e. that he held and wanted to put into practice a hypothesis that has occasionally been put forward, that a pope can separate the papacy from the Roman see.  But here are some notes about the state of the question.

In 1851, in the apostolic letter Ad apostolicae sedis fastigium, Pius IX condemned the view found in the works of John Nuytz, a canonist from Turin, who maintained that “nothing prevents the supreme pontificate from being, by the decision of some general council, or by the deed of all peoples, transferred from the bishop and city of Rome to another bishop and city” (the original Latin is available here, page 93.)  This condemnation was placed into the Syllabus of Errors, number 35.

That might seem to settle the question, since the pope can’t do more than a general council can do.  But perhaps Nuytz meant it in a conciliarist sense, i.e. he was perhaps thinking of a council acting independently of a pope, given that he also suggests that ‘an act of all the peoples’ (whatever that would look like) might also suffice.  Also, the opposite of ‘nothing prevents’ is not ‘divine law prevents’ but ‘something prevents’, so I suppose Pius IX could have had in mind simply that e.g. ‘respect for tradition’ prevents it, though that seems unlikely.

In the first draft of Pastor aeternus, at Vatican I, the second canon read:

If anyone says that it is not by the institution of Christ the Lord himself that blessed Peter should have perpetual successors in the primacy over the whole Church; or that the Roman Pontiff is not by divine law the successor of blessed Peter in this primacy: let him be anathema.

That is almost the same as the canon as finally agreed on, with one interesting change.  The words ‘divine law’ were moved and made into a gloss on the phrase ‘by the institution of Christ the Lord’.  Hence the canon as promulgated reads:

If anyone says that it is not by the institution of Christ the Lord himself (that is to say, by divine law) that blessed Peter should have perpetual successors in the primacy over the whole Church; or that the Roman Pontiff is not the successor of blessed Peter in this primacy: let him be anathema.

Some of the fathers had said that a distinction should be drawn between the law by which St Peter has perpetual successors, which they said was of divine institution, and the law by which these successors are the bishops of Rome, which they said was better said to be ‘of divine ordination’, i.e. that God had inspired St Peter to make the choice of Rome.

Bishop (not yet Cardinal) Pie, acting as Relator, was basically in agreement with this.  He said that the first law (perpetual successors) was of divine institution, and that the latter was of the institution of St Peter, disponente Domino, and that it is therefore a human law “which nevertheless is better and more truly called an ecclesiastico-apostolic law”.  He argued nevertheless that the canon should be left unchanged, on the grounds that it followed from two premises which are of faith, namely that St Peter has perpetual successors, and that (as Florence defined), these successors are, as a matter of historical fact, the bishops of Rome.  This seems like a bad argument, unless I have misunderstood it.

Anyway, a request was again made that the words ‘iure divino’ be omitted.  The next Relator, a bishop Zinelli, said that while it cannot be doubted but that St Peter transferred his see from Antioch to Rome as the result of a divine revelation (ex revelatione divina), as Innocent III says in letter 209 (PL 214:761), nevertheless, it had not been the intention of the drafters to condemn those who rejected this, but only to say that, given the divine law about perpetual succession, and the act of St Peter in choosing Rome, therefore the bishops of Rome are in fact these divine-law-promised successors (and hence that if someone refused to accept Pius IX as the successor of St Peter, he would be contravening divine law.)  However, he accepted that the canon as it stood was ambiguous, and said that it had therefore been decided to move the words ‘iure divino’ to the place that they came finally to occupy.

Islam is not simply a revolution brought about by Arabs who, bored of living under their tents, were stirred up by a gifted leader to make a sudden conquest of the most opulent cities of the East. Rather, God allowed the ancient enemy of mankind to have a special opportunity, and to choose an instrument by which he might lead nations astray, enslaving them by the sword. And so there arose Mahomet, the man of Satan, and the Koran, his gospel.

But what was the crime which induced divine justice to go to such an extremity, abandoning nations to a slavery of which we can still see no end? Heresy: for heresy is a dreadful crime which makes the coming of the Son of God into this world to be of no avail.  It refuses the word of God; it tramples upon the infallible teaching of the Church. Such a crime must be punished, in order that Christian peoples may learn that no nation resists the revealed words without the danger of suffering, even in this world, the penalty of its rash ingratitude. And so Alexandria fell, though it was Peter’s second see, and Antioch, where he had first been bishop, and Jerusalem, keeper of the glorious Tomb.

The tide was stopped in front of Constantinople, and did not immediately overflow the regions that surrounded it. The Eastern empire, soon to become the Greek empire, was given the opportunity to learn a lesson. Had Byzantium watched over the faith, then Omar would not have come to Alexandria, nor to Antioch, nor to Jerusalem. A delay was granted; it lasted for eight centuries. But when Byzantium had filled up its measure, then the Crescent appeared once more in vengeance. No longer is it the Saracen, who is a spent force, but rather the Turk. Hagia Sophia will see its Christian images whitewashed, with verses from the Koran painted over them. And this is the reason: it had become the sanctuary of schism and of heresy. [. . .]

It dared to penetrate even into the land of France. But a hard expiation it had to do for its boldness, on the plains of Poitou. Islam had made a mistake; where there is no heresy, there it can find no foothold. [. . .]

We shall stop here, having acknowledged the justice of God in regard to heresy, and the true reason of the victories of Islam. We have seen the only reason why God permitted Islam to arise, and why it did not remain an obscure and ephemeral sect in the deserts of Arabia.

We can remember also the words of Leo XIII in Exeunte Iam Anno:

The impartial and unchangeable justice of God metes out reward for good deeds and punishment for sin. But since the life of peoples and nations, as such, does not outlast their world, they necessarily receive the rewards due to their deeds on this earth.