Wow, these mockups really look nice so far! I feel like going with the silhouette art style was a wise choice, since it should allow you to get a lot more out of the NES's limited color palette. These mockups, particularly the first one in your post and the forest look like something that would be in an indie game, and they do stand out in a bold way, so I'm interested to see where this project ends up going. Since it somehow wasn't mentioned in the list despite, as far as I'm aware, being a Metroid staple, perhaps an underwater area would be nice to see? I'm particularly curious how that might look in this silohuette artstyle, and perhaps it could be merged with one of the other theme ideas you have, like the military base or the organism?