Question about fluidity from Extraordinary to Ordinary

For years, musicians have parsed the General Instruction for guidance on how to manage the musical problem in the ordinary form. To be sure, the GIRM is a reliable guide as compared with the really-existing liturgy found in most Catholic parishes. But what if the musicians want to push forward and manner more progressive than the GIRM seems inclined to support?

Some concrete examples: a polyphonic Sanctus, which is not forbidden by law but seems vaguely discouraged by the GIRM; introducing additional Sequences besides those listed in the existing liturgical books; using the Graduale instead of the Responsorial Psalm; dropping the Mysterium Fidei; using a Kyrie in addition to the Asperges; moving the Asperges to before the Introit, etc. All of these practices are known and frequent in some parishes advancing toward the ideal but they seem not to benefit from overt backing in the GIRM.

Now, a frequent mistake is to regard the GIRM as normatively prescriptive rather than descriptive, and I gather from Canon lawyers that this is just an error. The GIRM is to be treated as a description of liturgical events in the manner in which they are expected proceed, but not a manual of rubrics that is somehow binding in a normative sense. Still, we are left without much guidance unless we look to tradition on these matters, that is to say, we look to the extraordinary form of the Roman Rite, rather than seeing the ordinary form as somehow hermetically sealed off from liturgical practice through the ages.

My question concerns the implications of the Motu Proprio on these matters. It seems to be Benedict's impulse to encourage a kind of two-way borrowing from the extraordinary to the ordinary form. The USCCB is already saying that the old rule against "mixing rites" no longer applies since the old and new are to be seen not as separate rites but as two forms of the same rite. Doesn't this suggest, then, a greater degree of liberality concerning the ordinary form than we have previously supposed? What happens, for example, to the dubium from the CDW that ruled against new form celebrations employing the old form offertory prayers? Are we to see that as entirely overruled?

Other questions present themselves. How far can we go with this? Can Introibo ad altare dei be restored? The last Gospel? Can the extraordinary Roman Canon be now placed within the new form structure? What about the old formula for distributing communion? What are the boundaries?

My apologies if these questions are a bit naive. Maybe the answer is no, no way, that's absurd, can't be done. And yet Benedict's document seems to encourage a much greater borrowing in both directions. E.g, the MP suggests that new prefaces and new saints be added in the extraordinary form on the general principle that "the two Forms of the usage of the Roman Rite can be mutually enriching."

Perhaps another way to ask the question: what the prevailing norms for borrowing between two forms of the same Rite? Is there precedent or are we exploring a completely new world here?