I agree that a religion without a “goddess” is somehow incomplete. But I don’t agree that there needs to be a Christian dogma to that effect. On the contrary, as I note in my article on Schuon’s Mariology, “Colorless Light and Pure Air”––and as I suggest in another piece on my website, “Femininity, Hierarchy, and God”––the power of Divine Femininity is a function, at least in part, of its hiddenness and elusiveness.
You are right that the Orthodox would oppose any explicit attempt to position the Virgin on the same level as her Son, as a dogma of the kind you envision would seem to require. And opposed they should be. Nonetheless, the hymnography of the Church, notably the Akathist Hymns sung during Lent, speak of the Theotokos in ways any goddess would envy! I’m reminded of G. M. Hopkins’s poem about Mary, in which he says that “her presence, power is great as no goddess’s was deeméd, dreaméd”.
The iconography and architecture of the Church are powerfully suggestive as well. Behind the iconostasis, and behind the altar, in the apse of the Orthodox temple, there is always––or almost always––a huge icon of the Virgin called the Platytera, meaning “the one who is more capacious [than the heavens themselves]”. For unlike the heavens, she is able to contain God Himself. What need for a “goddess-dogma” when one is able to gaze upon this visionary equivalent, or rather more than equivalent?