The Draco project from Autodesk lets you sketch repeating elements, then use other sketched elements to define their replication and flow. It looks really promising:
We propose a framework built around kinetic textures, which provide continuous animation effects while preserving the unique timeless nature of still illustrations. This enables many dynamic effects difficult or not possible with previous sketch-based tools, such as a school of fish swimming, tree leaves blowing in the wind, or water rippling in a pond.
Nice! Looks pretty fun.