Still gotta finish bits of it but the basics are: we're going to cram even more shit into each room by using ceilings
By default, you won't see stuff that's on the ceiling. But, with a look-up button/verb/power, you'll be able to see stuff that's up there. For now, that's just lights.
Ceiling lights help fill out larger areas that might have open space that's too far from the wall-mounted tube lights.
Unless you're somehow really tall, you're going to need to stand on something to do something like change a ceiling light bulb. There are step ladders around, but eventually you'll be able to stand on chairs, stools, and maybe even the Mulebot to reach the ceiling.
Having some mounted heating/cooling equipment can also make it easier to manage the environment rather than drag a bunch of space heaters around. This is especially true for a map like Gehenna, depending on how well insulated the walls are.
Once pipes are sorted out, certain atmos equipment like scrubbers or repressurization air pumps can also go up there. Another possibility is fire sprinklers that feed from water or foam tanks. Tanks that, with enough forward thinking, you might be able to drain, or fill with welder fuel and plasma before you go set medbay on fire.
Finally, a pipe network for small packages (as opposed to the large, disposals-based mail system on the floor that would have much fewer chutes after implementation), but this is the last priority and might not even happen.