For me, they would need to focus on Disney Centric experiences.
How about:
Character meals - breakfast or lunch. They could do this as a free ticked event.
Disney Broadway shows - on a 4-7 night cruise, 1 actual broadway show.
Disney-fy all of the dining rooms - why not have BATB characters on a ledge around Lumiere'ss - better yet, make them animatronic and have them talk. Have Belle and the Beast come in and dance as a show in the middle of dinner. Every dining room could have special Disney touches so that it's not just a cute restaurant but a cute, DISNEY restaurant.
More deck parties and special events and theme nights. Why just Pirate night? Why not have Pixar night, Frozen night, Classics night, etc on all cruises or long (5-7 night) cruises?
Different experiences on the various cruises. If they are going to run the same itineraries, at least offer something different for each one. Have something individualized for 3 night, 4 night, 5 night and 7 night cruises. A specific Disney theme or experience that you can only get by sailing that ship, that many nights.
How about Star Wars Day, Avenger Day, etc more often? They own the brand, why limit the use to a few low season cruises.
More Disney specific foods - let's add in dole whips, mickey shaped pretzels and rice krispie treats, Epcot tasting items at the pool outlets that change day to day,
ESPN Sports bar onboard that has multiple tvs showing all major sports events covered by ESPN. Heck, lets add wings and nachos as free, consistent offerings available for a snack or lunch or casual dinner.
Better quality arts and crafts which aren't just paper and markers - how about design your own Disney mug or chef's hat/bag or beaded lanyard, keychain, etc.
More participation events that have small Disney themed prizes and enough availability/skill levels that more people can walk away with something at the end of cruise.
To me, the value of a
Disney cruise isn't in the cruise part, it's in the Disney part. I love cruising other lines, the only thing that would make me return to Disney is Disney experiences I can't get on any other lines.