Most importantly is it can be the SAME walk in closet. Meaning you can stock it with any number of goodies or traps, and summon the one you need.
I've been thinking ahead to when my Warlock will get the spell, and this is totally what I'm picturing. Since the spell lets you either establish a new one or connect to an existing one the first thing you do is create a handful of them during downtime and furnish each as needed.
So one demiplane is the Armory; a full compliment of mundane gear and specialized items. Need to outfit a dozen hirelings with arms and armor? Need two battering rams? Need 600' of rope? Open a door to the Armory and grab what you need. A 30x30x30 room puts any Bag of Holding to shame.
Another demiplane is the Guest Room; bed, chairs, rations, chamber pot, and some light reading material. Need to smuggle a squishy target past dangerous territory? Want to multiply the number of people a single-use gate or teleport will carry? Put them in the Guest Room, shut the door, and open it again once you've reached your destination.
A third would be the Workshop; sure, of course you've got an Arcane Laboratory at your home base, but sometimes you need access to one in the field too. Outfit the room with the sort of tools and ritual gear that don't fold up and transport well, and now you've got a mobile field lab.
The list goes on. Someone with the Sage background might want a reference library they can keep easy access to. A more socially focused caster might furnish a parlor room so they can always sit down for a cup of tea and a game of chess on a moment's notice. Be creative. You have an infinite supply of 30x30x30 rooms you can access from anywhere, stocked with whatever you can afford to put in them, as often as you can cast the spell.
Oh, and regarding ways to leave if you let the spell expire without exiting. The elegant way is Plane Shift back to your plane of choice, which should land you in relatively close proximity to where you want to be. The quick and dirty way is to use Banishment to banish yourself back to your home plane, since you're by definition not on it. That does leave the arrival a bit at the DM's whim, but come on, you're at least a 17th level caster. Fast travel is not that big an obstacle at that point.
