With the wind (rough seas), rain and snow in Western/Northern Europe, I don't think it will be a comfortable cruise.
Besides the weather, you also have to keep in mind daylight. Sunrise this time of year is after 8am, sunset around 3:30-4 pm
Northern Europe would be like intense around Christmas time. If the Magic were to stay over in Europe, they would have to do the Med and not N. Europe that time of year.
When there are enough ships, my bet is that the Magic transitions into a lot more time in Europe and other destinations that aren’t the Caribbean. Not full time because the Bahamas and Caribbean seem to be Disney’s bread and butter, but it will give them more flexibility with more big ships in PC and Miami