Increase the difficulty -- spawn more rain clouds! And to make the rain clouds more challenging, what about adding sine-like movement vertically in addition to the straight-line bullet movement?
Maybe change the firing controls, like in the spell shooter game, click the mouse and then shoot the fire in the direction of the mouse.
For aesthetics, maybe add a value-based animation to the colored raindrops -- Sine behavior, set to change Size (Magnitude 10, Period 2, for example).