Welcome to FR, Dan.
Actually it's already a doctrine of the Church and Our Lady already is the Mother of All Peoples which has the three aspects of Co-redemptrix, Mediatrix of all graces, and Advocate.
May we quote you on that? Various RCs on the forum dispute this claim when Protestants make the same statement.
Just for the record, there is a difference between a doctrine and a dogma. Dogma is solemnly defined by a Synod of Bishops with the approval of the Pope or can be defined by the Pope ex cathedra. Danny stated that Marian mediation and coredemption is a doctrine, not a dogma. So he's not claiming to be Pope :)
As noted above, this is a highly debated question amongst Catholic theologians. Like Danny, I would argue that it is Catholic doctrine (not dogma). See this article of Msgr. Gherardini for a brief explanation of why.
It's not a "new" doctrine. St. Irenaeus (died 202) says that Mary, as the New Eve, became the "cause of salvation, both to herself and the whole human race... the knot of Eve's disobedience was loosed by the obedience of Mary. For what the virgin Eve had bound fast through unbelief, this did the virgin Mary set free through faith." (Against Heresies, 3:22:4)
Pope Benedict XV wrote, "To such extent did she [Mary] suffer and almost die with her suffering and dying Son; to such extent did she surrender her maternal rights over her Son for mans salvation, and immolated Him insofar as she could in order to appease the justice of God, that we may rightly say she redeemed the human race together with Christ (Pope Benedict XV, Inter Sodalicia, 1918 A.D.).[the italics are mine]
At any rate, this discussion has been well-developed, as P-Marlowe noted, at this post.
God bless you all...
It seems many people are confusing the terms “doctrine” and “dogma.” Our Lady’s maternal mediation (Mother of All Peoples with its three aspects) is an official doctrine of the Church. It is not yet a dogma. It is valid to debate whether it should or should not be a dogma, but one cannot debate whether it is already a doctrine of the Catholic Church.
Also, to the other posters,
the title “Co-Redemptrix” does not mean that Mary is a goddess or is above Christ or is even equal to Christ. The “co” comes from Latin which means “with and under.” There is no hint at equality. The issue of “Co” meaning “equal to” is a faulty english understanding of the prefix. All of the virtues, grace, and privileges of Mary come from Christ. The title “Co-redemptrix” means literally “the woman with the Redeemer.”
This is not a new concept. It is present (called recirculation) in the Fathers of the Churc who said that as sin entered the world through a man, a woman, and a tree, so the act of redemption is accomplished by a Man(Christ), a Woman(Mary), and a Tree(the Cross).
God did not need Mary to enact our redemption, but he chose her from all eternity to be Mediatrix and Co-redemptrix with Christ, the Mediator and Redeemer.
969 "This motherhood of Mary in the order of grace continues uninterruptedly from the consent which she loyally gave at the Annunciation and which she sustained without wavering beneath the cross, until the eternal fulfillment of all the elect. Taken up to heaven she did not lay aside this saving office but by her manifold intercession continues to bring us the gifts of eternal salvation. . . . Therefore the Blessed Virgin is invoked in the Church under the titles of Advocate, Helper, Benefactress, and Mediatrix."