my own plane of existence 

could also be a good way to move half a million dragons maybe .

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?

Yes, Planar Shift or Gate are the most straightforward, although Banishment should work too.

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?

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. 

Can one survive in this demiplane or does the oxygen run out eventually? Doesnt say in the rules. 
For example if I want to store a living thing there, like an animal or monster or such, and they have food and water.

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

Could you use Dimension Door to escape? Asking for a friend...

Despite its to name, dimension door functions more as a worm hole. A doorway opens up, that ejects you at the point per the spell. Not viable for traversing dimensions.

I think, being that you create the plane, you decide its general nature. Like, I'm sure you could create a submerged demiplane to store things that had to be wet or a cold demiplane for things that that need to be cold. No reason you couldn't specify the air never runs out - it's an 8th level spell after all. 

What if:

Day 1. Cast demiplane, create demiplane, make a doorway in a wall. Nothingness outside it. 

Day 2. Cast demiplane, create wall-to-wall adjacent demiplane, make a new doorway in connecting wall.

Doublesized demiplane? 

Better usage is to just make a permanent teleportation circle in a demiplane.

Consider this: Demiplanes are 30 ft. cubes. You could fit an anti-magic field, a teleportation circle, and then some into one of these bad boys. Ideally, it can serve as a prison, as a safe room, as a treasure store, anything! The best part is that the only way that anyone is getting in is if they have the demiplane spell themselves. This spell is incredibly useful for Liches, or anyone else that has a few bodies that they want to keep hidden. The only problems are food and water if you're storing people. You could cast Geas on a low level enemy to keep the prisoners fed and whatnot, but the prisoners would have to be chained.

Spoon of nourishing

Or you could fill it with meatgrinders, spikes, blades, saws and acid, thus creating the Demiplane of Extremely Painful Death.

And then push the evil mastermind into it.

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.

Can you access something like a secret chest in your demiplane, because if it overlaps the ethereal plane then can you use the etherealness spell to get into it?

I go ahead and trap all of my enemies in there and let them starve.

Here's a fun use: Hour-long, two-way wormhole connecting the locations of two high level wizards who can be anywhere in the multiverse.

Wizards Fandalf & Bladirast are good friends. They know the nature and contents of each other's demiplanes well. Fandalf casts the Demiplane spell to create a shadowy door on a flat solid surface that he can see within range, choosing to have the shadowy door connect to Bladirast's demiplane, and he steps inside. He uses the Sending spell to send an interplanar message to Bladirast to open to his own demiplane, and Bladirast does this. They have just created an hour-long, two-way wormhole connecting two interplanar locations.

Fandalf's door (in the world) <—> Bladirast's demiplane <—> Bladirast's door (in the world)

Step one: cast demiplane on flat surface at the bottom of a large body of water.

Step two: open door and fill demiplane with 27000 cubic feet of water (= around 843 tons of water)

Step three: when you need to flood something, cast demiplane on a ceiling or high up on a wall and let it blast out.