That does not sound very viable at all. Winter is pretty cold everywhere in Europe, also in the Med. I think nearly all (if not all) cruise ships leave Europe for the winter season. Many leave in November and come back in March, so the European season is longer than what
DCL does currently. The cruise ships either go the Caribbean, South America or the Arab Gulf region during the winter.
My personal opinion is that the "second
Castaway Cay" rumour is the DCL version of the "fifth theme park at WDW" rumour. Comes up regularly, but is only wishful thinking by fans, no basis in reality. I agree with
@truck1 that there is no need for a second island and why would they invest in something pricey that is not needed?