Dunno. Personally I don't think it will be that much busier than normal.
Fantasyland has been in soft openings for a month now. Locals have already had the opportunity to visit. And I don't know that many people would have specifically planned trips to see Fantasyland immediately after the opening dates were released. We're also a few weeks shy of the start of winter breaks. I don't think they will start to see holiday crowds until at least the weekend of the 15th/16th.
Fantasyland may draw a greater percentage of people already on-site during the first days it is officially open but I don't think the crowds will be oppressive.