In the livestreamed playthrough of Chapter 1 conducted for Undertale's sixth anniversary, Toby Fox stated that he originally envisioned the game having a greater visual contrast between the Light World and Dark World, with the former looking "drab" and simplistic and the latter having more vibrant, detailed graphics in the vein of Chrono Trigger or Super Mario RPG: Legend of the Seven Stars.

Similarly, the Light World was at one point planned to be populated entirely by humans, and the Dark World entirely by monsters, with humans turning into monsters when they enter the latter. According to Fox, the idea was removed for "a lot of reasons," most prominently a desire to better distinguish the project from Undertale.
Contributed by game4brains