Its that one key in the central pillar room. You dont think to check there at first, so many people waste time going in circles and get annoyed by the iron boots switching while they are already frustrated not finding the key.
I dont think Metroid can be open world as a core gameplay mechanic is gating off progression behind upgrades, which is the antithesis to an open world ( at least how Nintendo likes to make them). They would have to revamp the formula significantly and at that point I am not sure it would feel like Metroid anymore.
Yeah I get what you mean. For decades Miyamoto has wanted to make Zelda games into a "box garden" -- a sort of small, living world and I think BOTW/TOTK have gotten closer to that ideal than any previous Zelda games. Not sure where they plan to take the franchise from here.
That finally came out? I've been waiting for ages since 5 came out.