Have you looked into a "Christmas Markets" European River Cruise ? I have not done one of them yet but it sounds great. If you go mainstream I would stay away from the family cruise lines like NCL, RCI and Carnival because of the # of kids on the ship. Celebrity or HAL will have less kids, better food, better service, more space per person and will not be a floating amusement park.
After reading your review of prior cruises I would suggest you stay away from the mega liner floating amusement parks. Sounds like you enjoy your relaxing time, Hard to relax with a bunch of kids yelling from the zip line or water park. Once you have been on a Celebrity Solstice Class ship your a bit spoiled. If budget is not a major factor consider one of the more boutique lines like Azmara or Regent. Your Captains Club benefit level from Celebrity should transfer over to Azmara.
Celebrity is my favorite line with the Equinox being my favorite ship to date.