My game's original "Only One" was to create a set of puzzles that required only one key press to solve. And that remained true in the final product. What did evolve though was how much I wanted to push that central gimmick. I came up with all kinds of weird puzzles by the end that still stay true to the gimmick but also (hopefully) changed the player's perception of it.
Unfortunately, some of the early puzzles I designed were too obtuse for players who can't read my mind (whoops!). The latter puzzles seemed more intuitive in testing, but my biggest regret of the jam was not revising those introductory levels (the answer to the ladder puzzle is "M" for anyone who wants to check the game out).