Alright, I will unashamedly admit to being in a venomously foul mood yesterday and tried for great constraint while posting (something I shouldn't have)
Thumper's Mom said, if you don't have anything nice to say, you shouldn't say anything at all.
Aside that, I really do hate Open World as a concept---because NO ONE DOES IT WELL outside of Rockstar. IMO.
Rockstar games; Grand Theft Auto/Red Dead Redemption, are "sandbox" games, yes, but in the linear story inside the sandbox is an open-world game. One which you can do anything to achieve your goal, and there are no really wrong answers.
There was a really hard car chase in Los Santos(?) of GTA:San Andreas that had you on a manhunt for some asshole. You had to run him off the road, chase him down, and put a bullet between his eyes. I tried and tried, to no avail. So I stole some buses nearby, parked them around the strip club he was coming out of for the mission, and blew him up with a rocket launcher. Open World allowed me the flexibility in execution of a mission.
Take Metroid now; and for argument sake, they decide to break down a few walls. How would this affect the gameplay? Well, here's a few problems I see off the bat; Power Ups.
One of two things happen; you gain all your power ups in the tutorial area (ie; BOTW) and then do whatever you want.
Or, you gain those suit upgrades and access new areas.
Well, how about the red doors? Or area access? If I can go anywhere, as suggested earlier; the exploration will be very boring.
The thing about Metroid as it is; or Metroidvania for that matter; is that you CAN'T go anywhere you want. It's the thirst to find out what's around the next map or corner, or fulfillment of finding a new power or rocket that makes Metroid so fun. If I can go anywhere, then it's kinda all the same. What am I accomplishing? I will continue to reference BOTW, because it's the only Nintendo I have to go on.
Now; I'll lay off the hate. And play the game here;
WHAT WOULD MAKE A COOL METROID OPEN WORLD?
-Multiple worlds
-Galactic Alliance access- Let's get our ship upgrades at outposts or a space station!
-Space battles; give No Man's Sky a taste of true coolness with space pirate shoot outs in the inky black starry sky.
-New planets to explore outside Zebes etc...
What ellllllse...
No, I can't think of much else. As long as the caverns are the primary areas of exploration and the planet surface is useless, great. Use cheap tricks like having a landing bay on a lava planet or whatever. Make the surface inaccessible​ and we'll be cooking.
Trust me, No Man's Sky killed the idea of exploring the surface of planets. It's extremely boring, even if you jazz it up.