YES!!! I've long advocated for more late night activities (and I really don't consider 11pm-midnight "late" - especially when on vacation). Could be as simple as scheduling more of the activities they already do (trivia, mixology, etc) later at night. They really need stuff to do after whatever takes place in the main lounge (Match Your Mate, other game-show type events, comedian/juggler/magician) at 10:30 which typically ends around 11:15pm. Why don't any movies start after that time? It's baffling that the last movie of the night usually starts 10:45 or 11pm. So it becomes an either/or thing. You have to choose between what's taking place in the lounge or the last movie. Due to their scheduling, both is not an option. And pools & hot tubs should generally be available 24/7.
I always thought the lack of activities and the ship being deserted by that time was a kind of "chicken or the egg?" scenario. Are there no activities because everyone retires to their staterooms by that time...or, does everyone retire to their staterooms because there are no activities? Well, the Wish indicated to me that it is more the latter since the Grand Hall fills up at midnight with people laying on the floor for the little kiss goodnight (I'm assuming this is still a thing). It shows that if guests are given something to do, even still at that hour, they will participate. So give us more!
At any time of day, I like a lot of what has been suggested already. The murder mystery thing over the course of the cruise OP mentioned sounds great. So do escape rooms, scavenger hunts and ship tours. On sea days, I'd like to see them do something in the WD theater that can draw a large amount of people away from the pool deck and help spread people out. I know other cruise lines do versions of popular game shows like Price is Right or Deal or no Deal. Something along those lines would be great.