Your rough sketch seems an 1:1 representation of @Grum999 plugin Buli Brush Switch
Where you define a brush to a shortcut, and this brush/shortcut has custom features.
My plugin don’t modify the brush itself, but define a shortcut to a ‘type’ of brush. For now it only has Erasers and Non-erasers, but maybe I could make more dynamic and expandable. Even adding a GUI too.