I also thought about a difficulty lvl based on the amount of money taken by the objects, the amount of enemies per wave and so. I wanted to have 3 different endings based on the amount of money left and make a real animation for the story. I made a system to create different waves in no more than 5 min, but sadly my obligations left me with no time to make more waves, and since it's only 20 waves I thought it wasn't much of a problem.