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.