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.
Could never afford it ... my point was ... I'd pay regular fare to be able to lie flat for the entire flight and you can forget giving me a TV monitor, entertainment, free food, just give me water, don't even bother with a blanket or pillow .... you can even stack me with three or four other bunks above or below me ... I don't even want a window ... I don't want to interact with your staff and I really don't want to have anything to do with anyone or bother with anyone for the entire flight ... just let me sleep in peace for eight hours, it's cheap and I really don't care or want any extras.
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.