This is fun! I'll chime in:
1. One way halls/stairs on the ships
--Too hard to enforce; people are going to take the shortest route. Also not necessary if people are wearing masks as they just pass each other briefly and are not exposed for long.
2. Designated hours that you can use your balcony (to prevent people on their balconies on either side of you, above or below you)
--Given "partitions" seems to be the answer to everything these days, and those are already on balconies, plus it is outdoors, this one seems unlikely
3. Mandatory vitamin D/zinc supplements
--
DCL legal would never permit this--they might make it available in the sick bay, but couldn't force people to take it, and not really shown to be effective against Covid anyway
4. BMI limits
--They may require people to certify they are not high-risk and sign some sort of waiver, but they are not going to start taking weight and height measurements at the terminal.
5. Elevator use by exception only, with sign up times, etc.
--This one I actually do think is likely. Not sign up times, but maybe certain elevators reserved for wheelchairs/strollers and other elevators one party at a time with spaced markers for folks to wait.
6. Quarantine times before and after embarkation
--DCL has zero way to enforce this. And the "before" part would be a killer because it would mean to pre-cruise travel or WDW stays--basically limiting themselves to locals. Various state/city governments may well impose mandatory quarantines after cruises once cruising resumes, though.
7. No daily cabin cleaning/turn down service
--Extremely likely, and very frustrating. I tip our room steward VERY much over the recommended amount for a reason.
8. Sign-up times for shuffleboard, miniature golf, ping pong, or basketballs
--Likely.
9. No one on upper decks during breezy conditions, since germs could carry further, faster
--That's a good thing, no? As they would blow off the ship?
10. Confinement to quarters during certain times of day
--Not at set times a day, but in case of a positive test on board, I think will be likely until they can administer rapid tests and contact trace.
11. No childrens' clubs
--I really don't see how they can sail if they do this. That and character meets are the primary entertainment for kids on the ship; and are essential if they want to still have adult only areas of the ship and adult-only Palo/Remy dining. Yes I know there are lots of childless adults who like DCL too, but that is not the typical DCL guest, and I really don't think they'd attract enough guests to justify sailing without families.
12. Two meals per day, to accommodate more distanced dining with an option for an additional charge room service for a third meal
--Room service is much less efficient and I don't think the kitchen could handle it on that scale. Plus, not sure it appreciably decreases the risk.
13. Lottery system for shows and movies with a guarantee of one show per passenger
--I'm thinking a combination of early sign ups for a portion with the remainder being available to reserve onboard first come first serve just like they do for other activities. But I could see them do a limit of one show that can be pre-booked and others have to be booked onboard, subject to availability. With a lottery, I'd foresee riots with everyone who gets "believe" instead of "frozen" mobbing guest services to complain.
Another implausible but funny one might be plexiglass between the lounge chairs on the pool deck. We'd literally be cruising along in a (plastic) bubble!
For me the real dealbreaker is going to be what happens when someone on board tests positive, which I think is almost inevitable. If they are required to end the cruise early and return to port, I'm not going to run the risk of that kind of disappointment. If it is just a quarantine for the sick person and traveling party, plus testing on board, I can live with that.
