The Orthodox celebrate the Dormition of the Theotokos already. It is not a big leap from that to the Assumption of the Virgin, and then you’re most of the way to the Immaculate Conception, especially because it’s moot anyway if you don’t accept Augustine’s formulation of Original Sin.
If there were otherwise a will for unity between East and West (and there definitely is not), I don’t think the Immaculate Conception would be the thing to stand in the way. The bigger problems are ecclesiastical and cultural.
What are you talking about? The Eastern Orthodox reject giving Mary's birth special status, you're just hand-waving doctrines with no explanations, entirely from a Catholic perspective.
I think “reject” is too strong. They don’t accept it. There are polemics by individual Orthodox who reject it. But I am not aware of any dogmatic rejections, especially since they haven’t had an ecumenical council at which to reject things.
There's no need for an ecumenical council to reject every single doctrine other confessions come up with. The Orthodox believe that the Fall changed human nature, which means that every human after Adam was affected. Since Mary was also a human, she was also affected. It was the Incarnation, the Crucifixion, the Resurrection, and the Exaltation which allowed human nature to exalt itself back to regaining communion with God. So the Immaculate Conception is rejected by the mainstream Orthodox faith and there is no need to have a council about it.
If there were otherwise a will for unity between East and West (and there definitely is not), I don’t think the Immaculate Conception would be the thing to stand in the way. The bigger problems are ecclesiastical and cultural.