The thing is, even if you have paths that only lead to the night and day(?) areas, and even if you only use the limited tilesets you are already using, you can create some great screens and maze like areas that go up in to the sky, or down into the water/ground. I think you know how to make each screen original and not simply repetitive, so I think you'll keep it interesting. The thing is just to add enough area to explore that the player feels like they are experiencing something spacious, rather than a small little level. Even a tight and small world needs to feel spacious just so you feel like you've done something by playing the game.
And just my impression, I never would have thought of the first, colorful area as day. More like a dreary or rainy dusk. Wait, did they change color to symbolize the sun going down?
I always like you games yohji.
I look forward to the expansions and the deeper story line.