I really like this one! If there where more levels I would have continued playing. I think with all the character in the game you have enough to build very challanging levels. So my preference would be to add more levels instead of new mechanics. Maybe you could think about letting the player choose the difficulty: tutorial (basically what you have right now, but a bit easier), easy, medium, hard, ...
Another Idea could be to create a random level generator that can create levels of different difficulty ... than you would not have to create them all by hand. But I am not sure how realistic that is...