I really like this idea. I've wanted to try Costa for awhile now but they left US shores a few years ago and haven't returned. But the idea of being on a Costa ship with a Carnival crew is VERY intriguing for me. As has been posted, these are new, beautifully designed ships so mixing that with the fun of Carnival will be very interesting to watch. Even though selfishly I wish one of these was coming to Florida and sailing the Caribbean, I will travel to sail on one for the unique opportunity. I kinda feel like this is a reaction to MSC's growing presence here.
Beyond that, there is mention of a drydock before the handover. I doubt they'll change much but I can definitely see some of the 'Fun Ship' eateries being added to appeal more to the US taste and market. Otherwise, I say leave them as they are! Following closely.