It has been discussed (see mixer brush discussion above) as well as in a thread on the smudge brush. I’ve started a longer term project of creating the mixer brush, because despite the fact that the smudge brush can do just about everything the mixer brush does, there are some things about how the current smudge engine works that I don’t like. Since I’m pretty sure my changes will affect existing brushes, I’m just making a new engine, and since I’m making a new engine, it’s going to be more closely modeled in UX to the mixer brush. I’ll also make sure to include the dirty brush option while I’m at it (both options of saving a color and saving a whole dab between strokes).
But like I said, it’s a longer term project, so hopefully sometime this year, but probably not in the next few months.