Well it's open in the sense that by the end you can go anywhere at anytime and do anything the game allows you to. The core of the Metroid games is in progressively unlocking more and more of the maze. So the world isn't seamless and none of it is open from the start.
But to turn metroid into a...
Why not opin walrd? Everything else is. It's the go to thing to do when you haven't the slightest clue wtf you're doing.
Metroid has always been open world in a sense anyway. The Metroidvania style of open world is just a more structured open world level design.
IMO Metroid could stand to have...