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.