None of these ideas are original, but to bring my family back they would need to add:
1. New itineraries. There are only so many times one can go to the same Ports of Call and, while I'm definitely the exception, I cannot stand
Castaway Cay, so having that as a requirement on all cruises as one of the ports is a major negative. This is the single biggest factor driving us to other lines; we'd like to go places other than Nassau, St. Thomas, San Juan, and Castaway Cay. On our next cruise that we booked to replace one we just canceled on
DCL, we're going to Roatan, Honduras, Belize City, and Cancun, and that variety is interesting to us.
2. New shows, and I like what several posters suggested about using truncated versions of their Broadway shows. The musical revue shows are exceptionally cute once, or even twice, but I simply cannot sit through them anymore and neither will my children or wife.
3. More dining options. The rotational dining is very unique and each restaurant is nicely themed (although outside of Animator's Palace, they all remind me of somewhat generic ballrooms, but in a pretty way), but it also forces you into a system and those restaurants take up space that could be used for other, specialty restaurants. Likewise, the menus offer nice diversity for the first few cruises, but after several years of the same menu, it gets stale and old. All mass-produced food on any mass-market cruise line will always be difficult to prepare well, and I don't think that DCL's is worse than any other mass-market line, but I also don't think it's any better, and for the premium I think they could work on that.
4. This one is weird, and I know it's not a typical opinion, but I'd actually pay more money for less obsequious service. Honestly, I find the constant hovering and wheedling attention to be off-putting. I do not want to be best friends with my servers, cabin steward, or others. I prefer exceptional service in the background, rather than desperately-seeking-approval service in my face.
As to the look and design of the ships, I will agree that from the outside they are quite attractive, but given that I'm sailing on the inside of the ship, looking out, it does not much matter to me. The interiors I find to be fairly similar to other mass-market cruise ships, and while lots of dark wood and brass is not my personal design aesthetic, it's classically nautical and pretty much what I expect. The children's clubs are very nice, although again after multiple cruises my children have gotten tired of the same activities, same games, same programs, etc. That's not a criticism of what they offer, however, but it's not worth doing a fifth or sixth or seventh time, at least for our family.