This feels like a 5th level spell, more of a perfected form dimension door.

Has there been an official ruling on the 'environment' of this spell?

We've always agreed at the table that you're effectively 'making' a brand new plane, and it is a level 8 spell, so yeah, you can decide the environment.

The last one I made was essentially a small 'hollow earth' made of rock with a central pillar that the party enchanted to glow with a Permanent Daylight spell, and we arranged for a Druid to come in and cast Plant Growth once a year, and stocked it with a small water reticulation system via Permanent  Unseen Servants and a Decanter of Endless Water, and then slap Mordenakien's Private Sanctum for a year in that bad-boy and you've basically got an impregnable fortress to anything that isn't a Demon Lord-tier threat.

So we had a cave lined with lead where the air was always pleasantly warm, there was fresh drinkable water, imported edible mushrooms from the Underdark and a chest that had the functions of both a Bag of Holding and the Gentle Repose spell and furnished one corner of the 'cave' with a small 10-by-10 room with a bed, a wash-tub and a small amount of books, spare clothes and a basic kitchen. If we needed to lay low, or stash a NPC somewhere anything other than another high-level spellcaster, God or Demon Lord couldn't find them, bang, into the hole they went.

I mean, make a bargain with some Celestials to hang about in the place for a few months at a time and they'll eventually Consecrate it just to absolutely ruin a bad guy's day if they somehow managed to figure out how to get in and whoops, say hello to the Lantern Archon Firing Squad...

Trick the dragon into humanoid form, pull it trough the door (or create the door under the dragon) and use forcecage in the front of the door to maintain it there.

Yep this is gonna be in my interesting Drifter Class
Drifters are known for their great soul and spirit, hopping through planes, terrains, and teleporting to a known place as home. These humanoids learn the rightful path and art of their Drifter Order and assemble a symbol of spirit.

Cast sequester on allies in advance and store them in demiplane set to release them next time demiplane opens, you could essentially bring a small army with you wherever you go, sneak into places with only a couple people then release your allies, id also personally use this to store undead, would make a good destraction in a pinch

just fill the room with nalfeshnees

A bit of brainstorming...

I'm inclined to agree on the point of oxygen, though it would be hard to argue that if unlimited oxygen is an option, unlimited water is also an option, which could potentially be used as a death trap:

* 1st player readies action to cast Thaumagturgy to open the door when it appears

** 2nd player readies action to cast dispel on the door once the creature falls through

*** 3rd player summon the door on ground beneath the medium sized creature.

I admit, it would not be easy to pull off as the target could just step off the door on it's turn if not timed right, but it's do-able.  I would be happy if my players figured this out and pulled it off. 

I would probably allow a Dex save to grab the edge if it was used on a player, so I would likely allow it for the target as well - though it would be forced to drop anything in its hands and be vulnerable to having its grip targeted on subsequent turns as it held on.  If the room was filled with water, the DC to climb out would be lower than if filled with air...

Though if filled with acid or magma... hmm.  Lots to consider on what's allowed to be in the demiplane.

Quote from Dottn >>

If you've somehow managed to trap yourself in a demiplane, how'd you get out?

Casting Demiplane again would connect to another demiplane, so you'd need planar shift or something?

 Each time you cast this spell, you can create a new demiplane, or have the shadowy door connect to a demiplane you created with a previous casting of this spell. Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead.

Quote from HelloThere5 >>

Quote from Dottn >>

If you've somehow managed to trap yourself in a demiplane, how'd you get out?

Casting Demiplane again would connect to another demiplane, so you'd need planar shift or something?

 Each time you cast this spell, you can create a new demiplane, or have the shadowy door connect to a demiplane you created with a previous casting of this spell. Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead.

 So you can go to another existing demiplane, but how do ya get to someplace that's not a demiplane at all if inside one?

If you were to store a clone in here would it not be accessible if you died on the material plane?

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Quote from OwenJNick >>

If you were to store a clone in here would it not be accessible if you died on the material plane?

Quote from OwenJNick >>

If you were to store a clone in here would it not be accessible if you died on the material plane?

 It says nothing Within the clone spell description about being on the same plane of existence, only that the cloned soul need to be able and willing to return to their cloned body

So, say one were to break through the walls of a demiplane; what'd be on the other side?

What exactly does the "nature" of a demiplane refer to?

>Create demiplane

>Walk in and cast magnificent mansion

>Tell army to walk through demiplane into mansion

>teleport into hostile territory

>open demiplane

>enjoy the decimation of your enemies

Quote from AlabasterSlim >>

Quote from Tasfalen >>

So essentially this is a planar storage unit that you can open the door once a day and the door stays open for an hour for you to add new stuff or pull stuff out?

 Pretty much. Or a planar jail you could push enemies into. 

 My thoughts exactly use this, true polymorph or wish, and some dimensional shackles and you can permanently seal Tiamat 

Quote from Maverik6464 >>

can you dismiss this early? like cast the spell, push someone in, then dismiss the door?

 Use dispel magic with your ninth level slot

I think it would say so if it had a limited air supply (like the bag of holding or the portable hole)

Makes an excellent jail for creaures that have no means of planar travel.

Quote from Terrymln >>

I love the idea of combining Demiplane with clone of an NPC or a player character if the GM allows casting Demiplane to escape from inside(As a GM i do allow). It becomes a a perfect safe place for the clone to grow and be stored along with backup items and spellbook.  As a bigbad, even if the party Kills them they can come back and would be a big surprise if a younger stronger version appears much later in the campaign.  It also puts the party on a time table to kill the badguy before his clone matures, so 120 days or less from when they find out.

Quote from ethana1275 >>

Quote from OwenJNick >>

If you were to store a clone in here would it not be accessible if you died on the material plane?

 

Quote from OwenJNick >>

If you were to store a clone in here would it not be accessible if you died on the material plane?

 It says nothing Within the clone spell description about being on the same plane of existence, only that the cloned soul need to be able and willing to return to their cloned body

 I don't know any mechanical rules about this... but souls are typically able to cross dimensions.... how else would the afterlife work? So your demiplane can be your own personal stop before the afterlife to pick up a new body.