Consider the following scenarios.
1. Banished from a portable hole, portable hole is destroyed.
A portable hole is described as a ten foot deep, six foot diameter extradimensional space. Suppose I jump into my portable hole after spreading it out on the ground, and I am followed by an enemy. Once we are both inside my portable hole, I cast banishment:
If the target is native to a different plane of existence than the one you’re on, the target is banished with a faint popping noise, returning to its home plane. If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied. Otherwise, the target doesn’t return.
My enemy is banished to its home plane. Next, I climb out of my hole, get a safe distance away, and toss in my bag of holding:
Placing a bag of holding inside an extradimensional space created by a handy haversack, portable hole, or similar item instantly destroys both items and opens a gate to the Astral Plane.
The portable hole is destroyed, and finally I break my concentration on banishment before the full minute has passed.
2. Banished from a rope trick right before the spell ends.
Rope trick says:
an invisible entrance opens to an extradimensional space that lasts until the spell ends. […] Anything inside the extradimensional space drops out when the spell ends.
So I cast rope trick while I’m being chased, and my pursuer pursues me into my little rope trick room, where I am patiently holding banishment. I banish my pursuer, climb out of my rope trick room, and and cast dispel magic on the rope:
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends.
Again, no space to return to as I break my concentration on banishment before the one minute is up.
What happens to the banished creature when banishment ends? Banishment is very specific that the creature returns to the space it left from. Both the actual extradimensional space and the 5 foot square space the creature previously occupied is gone, as well as all nearest unoccupied spaces. What happens?