What about giving points (or more time, or any power-up) if you make something, like making a circle around a ball or not moving for a number of seconds. For sequel, Particle 2 rate up!
PS. Should be just a little of friction, because the speed of the balls keeps accumulating, and the fact that it even has more balls in the same space, it causes an exponential growing difficulty. Nice job 5/5!
Players must have an attention in how the cards are going to die or move towards the left. That way they can see when to get, for example, an Anti-Air card with 1 turn to wait for its activity.