I think Bishop Fellay has always said Rome had a "Rolls Royce" canonical structure set up for them. When he met with the Pope in 2005, the Pope turned to Cardinal Hoyos to ask where they stood and Hoyos said everything was set for the Society to come in and the papers regarding the Canonical structure were put together and being studied by the Roman Office for Canon Law!
Bishop Fellay, of course, put on the breaks, quite surprised, and brought up the issues that still remained. During the course of that conversation BXVI called Archbishop Lefebvre "The Venerable Archbishop Lefebvre" and "This great man of the universal Church." Needless to say Bishop Fellay was floored.
The Pope has also said his two biggest regrets as Cardinal were the way the Fatima third secret was handled and the breakdown of negotiatoins in 1988 with ABL.
I think this Pope put his money where his mouth is in showing by actions, as well as words, that he takes the Society seriously and wants them in the "official" Church. Bishop Fellay laid out 3 pre-conditions and the Pope met them ALL. Say what you want about BXVI, but can you IMAGINE Paul VI or JPII meeting ANY of these conditions?
They were, of course, the freeing of the Mass, the lifting of the "excommunicatoins" and doctrinal discussions. I think it is a miracle that the Pope met ALL THREE before ANYTHING was required from the Society.
The big X factor are the doctrinal discussions. Nobody really knows what the situation is or how they will end up.
In the end, I think a Papal statement or Motu Proprio, as the fruit of these discussions would be ideal. I envision it including a list of "errors" or false "interpretations of VCII". For example, "if anyone would interpret "subsist" in any manner that would deny the Catholic Church is the Church of Christ, let him be anathema" and so on.
Even if it simply closes the doors on the worst ambiguities of VCII, I think the Society will have succeeded and done a great service for the Church. Rome has not definitively interpreted any of these ambiguities which have served as fuel for liberal Catholicism. Cardinal Ratzinger tried to some degree in Dominus Jesus but still left the door open.
In any case, I think the personal ordinariate would be a great thing. It puts the Society outside the control of the Bishops. That is HUGE. They could continue just as they have been, the only difference being they are inside the "official" Church, thus taking away ALL of the "full communion" liberal and Neo-Catholic technical arguments of "licitness" and of that crap. THEN they would be FORCED to get down to brass tacks. The Faith! They can't use the tired old "schism" and "disobedience" canards. Then the fight goes to doctrine and they will HAVE to see the Society's position as a legitimate one they cannot IGNORE or DISMISS as those on CAF and Neo-Cath apoogists do with impunity.
And WORST CASE let's say Rome starts mistreating them or silencing them or interfering with them. Guess what? They keep doing what they've been doing and if Rome wants to start sanctioning them, who cares? Has the Society not been down that road before? And because Rome knows this, I think they will be VERY hesitant to mess with the Society once they are "in". I think they will be left alone. I think it is a win/ win for the Society. They would have little to lose and a lot to gain.