How about just bunk beds with seat belts or some kind of restraint and a paddled ceiling inside your bed space.
The next dumb thing to having an completely open floor space is in installing seats and normalizing the idea that people should sit in them for 8, 10, 12, 14 hours periods.
The only time we need seats with restraints is in the ten minutes after take off and the ten minutes when landing .... the rest of the time, I would prefer if I just slept the entire time.
I'd give up TV, a monitor, music, a window, free food and drinks if airlines just gave me a bare bones option of just being allowed to sleep flat for the entire flight.
I unironically think this is how plane travel should be. Slide me into a tube and gas me asleep. No peanuts, no crick neck, no risk of hijacking, and no praying and screaming when we slam into a mountain at 900km/h because the pilots had a bad day.
There are several ways they could do stacked beds or offset-stacked reclined seats that would not only be more comfortable and provide more room for the passengers, but also allow them to cram more people in the plane.