Will we ever see collision detection on foliage? That should be great.
You mentioned the painter. The idea is good. You could incorporate in one brush multiple foliage models, like blades of grass, or even trees and grass.
I think there is an equation for the correct dispersion of grass around trees
One good think to have in the painter tool is the random function. You do not want the vegetation elements to be painted in the same pattern. A random function for the distribution would be nice.