Octane on 29 September 2015
Don't expect Retro Studios is working on Metroid. We know that most of the staff that worked on Metroid Prime aren't with Retro Studios anymore. So in short, the Retro Studios of today has little to no experience with Metroid. Tanabe also confirmed that Metroid Prime 4 isn't in development. Unless Sakamoto has been working secretly on another main series Metroid game, I highly doubt that there's something coming anytime soon.
