Wouldn't it be cool if you could select a brush, hold down CTRL (for instance) and select another brush, and another brush,etc? Each selected brush would "override" the settings from the previous brushes to create a composite brush. Then you could try it out and even save that as a new brush. The only technical change would be some GUI logic and a way to "merge" brush settings.
Another idea is a randomizer interface to create new brushes. This is a feature found in musical instrument software tools provided by Korg (and I'm sure many others). The idea is you have say, 50 different variables to produce sounds, why not start off with a completely random set of variables and see if it sounds good. If not, click a button and get a new set of variables. Like rolling the dice to create your character in a role-playing game. Wouldn't that be a neat way to discover brushes? Maybe the interface would let you lock certain variables like brush radius, or provide thresh-holds to avoid HUGE brushes that make your computer crawl. It would also be nice to limit the number of randomized variables. Say, only randomize 10 settings at a time, or only these particular settings. This could basically be its own standalone GUI program/window and wouldn't affect anything else as far as the brush engine.