Have any uniquely Marian devotions developed after Vatican II that didn’t exist prior?
Off the top of my head, I’d say that official Church approval of the luminous mysteries of the rosary postdated Vatican II. These mysteries were authorized by Pope John Paul II in his apostolic letter, Rosarium Virginis Mariae. Perhaps participants at the Catholic Answers Forums can offer further examples. You might post the question in the Spirituality Forum for their response.