Sailing this year is probably out of the question. But, I would think the DREAM would be first unless they all changed their itineraries to just
Castaway Cay/Bahamas. I would take a cruise to just CC. They could sail there and let the Dream have 1/2 days, Fantasy 2/3 days, and Magic and Wonder have 2 days each. The schedule could easily work out on 3-7 night cruises. They probably would need to make the Fantasy 5 nights in length (same for Magic and Wonder.
There would never need to be 2 ships at Castaway Cay at the same time. 3 night cruise Dream: all day Monday, 5 night cruise Fantasy: Tuesday and Wednesday, 5 night cruise Wonder: Thursday and Friday, and 5 night cruise Magic: Saturday and Sunday
They could do this until the restrictions to other ports are lifted.