Doesn't take much to make it longer, though thinking up a bunch of stuff helps.
It took me less than a day to make it, I could probably make an open world, nothing would change over time because you don't get varables to work with, but I could give some lore with objectives with attached dungeons, which you could go though and get an ending (A evil beast in a cave: go though cave, solving puzzles, and in the end you get an ending with the world being a better place, or some rumor about a tower of the gods and becoming one by getting to the top: go though tower dungeon (PUZZLES) and get an end of becoming a god or a choice of not.)
Edit: Either I'd make the open world something like a 8x8 of areas, or a bunch of islands of varing sizes conectend by ports.
Edit: And I remembered how anoyying jugling three colors is, I'd proabably make the areas with water mostly water because I couldn't have things like trees in the same areas with water.