In the game's files, there is an NPC resembling what appears to be a Headspace version of Sunny wearing a flower crown. The developers most likely used this NPC for various testing purposes during the game's development.
In the 2017 trailer, there was originally an option to look through the keyhole when interacting with the door in White Space. Doing so will lead to short glimpse of Kel smiling back at Omori. This feature was removed from the full release for unknown reasons.
According to Omocat, Mari was originally planned to be an ice cream princess during the game's early development stages. This idea was conceptualized around the same time when Orange Oasis was supposed to be required for story progression.
In the game's files, there exists an unused set of sprites of a monochrome Basil. These sprites were used for testing purposes in one of the debug rooms titled Dev Room C. When speaking to any of them, the Basil NPCs will simply say "Waka" in response.
In the game's early development stages, the developers planned that the player could change Sunny's gender between male and female. This feature would be scrapped later, most likely due to how it would require much more work to add additional sprites for Sunny's overworld animations.