Question on this: would frozen water (ice) count as occupying the space? It is solid, but if it is very clear (as clear as water) and you could see through it, it fulfills the definition of "unoccupied" as you have described them.
Ice is not a distinct thing. You cannot have an ice. You can have a block of ice or a cube of ice, but "ice" itself wouldn't be a distinct object.
So could you Misty Step through ice? Could you Misty Step into ice?
(sorry for snipping the text, it's just to avoid copy/pasting the reply/answer chain)
Considering ice is solid, I'd say it's like a wall, a block of stone, or any other terrain you can't walk into, so to me ice counts as an Occupied Space.
BTW, this rule from from the PHB is also interesting for the thread:
What Is an Object?
For the purpose of the rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone. It isn’t a building or a vehicle, which are composed of many objects.
(sorry for snipping the text, it's just to avoid copy/pasting the reply/answer chain)
Considering ice is solid, I'd say it's like a wall, a block of stone, or any other terrain you can't walk into, so to me ice counts as an Occupied Space.
BTW, this rule from from the PHB is also interesting for the thread: