I was part of the 90% travelling with kids, but I still really enjoyed time in the adults only areas. My kids WANTED some time in the tween/teen clubs and after all, isn't great kids clubs one of the draws for
DCL? So when they were there, I enjoyed some time in the adult areas. I would want that to get any smaller.
Some areas of the ship were definitely under-utilized, but that is great too. Such a big ship with so many people and still some empty areas. I would say, better wind control on the upper decks and more optional (Maybe retractable?) shade on the top decks would be great. Some areas were empty much of the time because it was just too Windy/Sunny to do anything, even just lie there, and thus, those areas went unused.
Remember too, much of the reason that the pools on the ships aren't bigger is due to Physics. So much weight, so high up, but maybe more modern ships can cope with that too.