I actually think Demiplane is one of the neatest spells there is. My take on the spell is that the plane created is permanent, it is merely the door to it exists for only an hour at a time when the spell is cast. It's like your very own trans dimensional safe deposit box, and only you have the key. One of my players found an artifact that he had to hide until he could figure out what to do with it. Knowing there were some bad people looking really hard for it, he put Sequester on the item, then stashed it in a demiplane. Thinking about it I figured that's as good as it could be protected and without specific knowledge of how to access the demiplane it would take a wish level effect to locate it.
Then I got to thinking what if you stacked demiplanes. Make one. Then make another inside that, and kept nesting them. If I ruled that further castings within such a plane could be modified to simply expand the existing plane, you'd be making your own little world 27,000 cubic feet at a time. Given enough time you can carve out a nice little private world for yourself. Portals to the elemental planes could be used to bring in what would be needed to make it look like a proper world. Additional protections using castings of Private Sanctums, Forbiddance, and others can give you a great deal of protection and privacy. Cool spell. I love high level casters.
Then I got to thinking what if you stacked demiplanes. Make one. Then make another inside that, and kept nesting them. If I ruled that further castings within such a plane could be modified to simply expand the existing plane, you'd be making your own little world 27,000 cubic feet at a time. Given enough time you can carve out a nice little private world for yourself. Portals to the elemental planes could be used to bring in what would be needed to make it look like a proper world. Additional protections using castings of Private Sanctums, Forbiddance, and others can give you a great deal of protection and privacy. Cool spell. I love high level casters.