Yeah, maybe a number of stage "packs" or "worlds" (maybe "floors"?) or something that each introduce a new mechanic in a tutorial level or two, then include a handful of stages that are harder and incorporate mechanics from previous floors? Just an idea. :)
Great job including opening & ending cutscenes, by the way. Those make it feel much more complete as a game, and a lot more satisfying to finish! I can tell you spent a lot of time considering the player's overall experience.