Hi,
I'm not sure if this suggestion would match MyPaint philosophy, anyway...
What about having the possibility to apply effects (ie darken, lighten, sharpen, smoothen, perhaps personal convolution filter, emboss, etc...) with the brush?
I imagine the effect feature as a switch (or a tab) on the color selector, rather than a dedicated brush; ie each of the brushes could be used to apply either the selected color, either the selected effect.
Top of the top would be the ability to create its own effects with an effect editor, similar to the brush editor, so that we could share effects with the community...