This is a follow on from a discussion on IRC with @Campbell Barton (campbellbarton).
Recently, contextual menus menus were added to the paint modes. However, inside the paint modes we hardly have any useful operators to add here.
Instead, we could add common brush controls here, as many painting apps do.
To start, we could just add a simple list of sliders:
Later on, when the brush picker is more fleshed out, we could add it here too, like so: