I have the feeling danmaku could be much more. In the present time, it's essentially splines with a bit of derivative hacking. Almost every pattern from ZUN or CAVE's is based upon that.
I think danmaku should be considered like music for the eyes. Using that metaphor, I'm not sure how patterns in patterns (rotating/expanding arcs of circles of bullets, reflection, aimed bullets, cascading bullets, etc.) would map to melody, harmony, and rhythm, but it is obvious we're lacking something. I've seen some of the most complex patterns Danmakufu users produce, but they generally looks unharmonic.