The demiplane created by the demiplane spell "...appears to be an empty room 30 feet in each dimension, made of wood or stone."
Are the outer surfaces actually wood or stone, or do they just appear that way?
Can you dig into the walls? What happens if you do? Can you effectively expand the demiplane by digging the walls, or maybe disintegrating big chunks of wall? Or does breaking a wall reveal some weird phenomenon? Or does something else happen?
Can you at least hammer nails into the walls to mount pictures, torches, ladders, etc.?
Do walls made of wood burn? Can you cast stone shape on walls made of stone?
Oh snap, I hadn't even considered air supply...
But, the idea of a latrine that dumps your poo directly into the astral plane? Priceless!
The answer to digging through the walls is going to be some form of "it doesn't get you any more room." There's nothing outside the room. That's the entirety of the plane, meaning it's the entirety of the universe in which the room exists. How it works doesn't really matter. The could plane wrap on itself Portal-like or hall-of-mirrors-like, so that if you disintegrate the left wall and a hole appears in both the left and the right wall and travelling through the left wall leads through the right wall back into the room. Or beyond the walls is a black or silvery nothing that can't be entered, or an open sky that can't be entered. You don't get more than 27,000 cubic feet. Physical space just ends, and the universe of the demiplane is no bigger than that room. It's up to you to decide if space is curved or flat. Personally, I'd make the floor and ceiling flat silvery nothingness that can't be entered, and the walls Portal-like and wrapping because that seems fun.
In general, I would assume that the air supply is continually refreshed. That seems well within the power of an 8th level spell (after all,
rope trick appears to do it at 2nd level, and
magnificent mansion can certainly do it at 7th), and, IMO, the DM shouldn't be looking for gotchas to spells of this level (beyond
wish, of course). Let the spell be as powerful as an 8th level spell. Let it accomplish things that an 8th level spell would be capable of. It's competing with
clone,
control weather,
earthquake,
mind blank,
tsunami, etc. The power of the spell is in creating an array of reusable 30 foot rooms. The details like whether there's light or atmosphere are trivial compared to the rest of the spell. I'd tend towards giving the PCs the benefit of the doubt, and if they find a truly egregious abuse then change your mind. It's not really a problem if an 8th level spell replaces
rope trick and
tiny hut, or semi-replaces
magnificent mansion.
I would say that the walls function exactly like normal stone or wood and are no more than 1 foot thick. The walls can be damaged, destroyed, or otherwise used, and they may either regenerate themselves or remain destroyed when the spell is recast. I don't think it's important which you chose, so you might let the caster decide. I would say the demiplane can either be illuminated like
tiny hut is (i.e., a light that can be toggled at the occupants' decision), or can be permanently lit or dark, at the caster's decision. These are all very minor details in the grand scheme of creating a demiplane.
Personally, I wouldn't let there be a trash hole to the astral plane, but I've seen players abuse such things. There's a reason that the
bag of devouring is considered a beneficial magic item to experienced parties. Being able to permanently dispose of creatures and objects without destroying them is very powerful.