I would like to be able to select several brushes in the Brush palette, then press a 'make brush group' button on the palette.
I would then be able to choose this brush group as if it were a brush. Drawing with this brush group would choose a different brush in the group at random for every new path I created.
I might have a 'brush group options' dialogue box wherein I could choose the relative frequencies of the different brushes in the group, switch it between random choice and cycling between brushes in order, give it a name, and... whatever other features seem to need to be in there once it starts taking shape, really.
Ideally, I would be able to have a brush in multiple groups. I would also be able to have brushes of different types in one group - if I wanted to mix an art brush with a scatter brush, for instance, I could.
And ideally, altering the brush group would alter existing paths drawn using the brush group. This might be a choice similar to how changing the settings of a brush that's in use asks if you want to apply the changes to all existing paths that use it.
I've been dabbling with a more 'natural media' look in Illustrator. For my purposes, art brushes seem to be working best, but I'll often start to notice the same repeated features of an art brush sticking out of my drawings. If I could make, say, ten "dry brush" brushes, and have Illustrator constantly swapping between them, this would make the illusion of real media a lot easier to keep up.
Similarly, I use a lot of scatter brushes for things like trees or grass. It would be useful to be able to have several different 'leaf' scatter brushes and just scribble in a few overlapping strokes that automatically pick between them.
I would also be able to create Graphic Styles that call for the use of a brush group on a stroke instead of a single brush.