It looks like multibrushes will be added at some point in the future. I wanted to suggest allowing the additional brushes to be added or subtracted from the original brush.
-You could have a normal circular brush (positive) with a smaller circle inside of it (negative) and the end result would be a ring shaped brush.
-A positive circular brush and a negative circular brush offset from the 1st. This would create a crescent shape. If you can scatter the 2nd shape, then you could end up with a random variety of crescents. But it would be cool to control them by stroke direction or tilt/azimuth.
Where this would get really cool is when you add soft edged brushes. This would allow you to create "wet edged" brushes.
And lastly, you could have the additional brushes add or subtract from 100 to -100%, respectively. Then you could have regions of varying opacity within a brush shape.
I can already envision myself creating some REALLY complicated brushes.