Something like this would be really nice, but I don't think that just using direction is enough. I think that for 2.8 we should have a kind of popup tool that allows dragging various things. This needs to be properly designed, though.

Having sliders come up is nice, but *most* of the time precision isn't needed, and a popup adds extra clicks. The current brush resize is fast and a real pleasure to use, the main reason being is the lack of a slider UI. One could really go crazy and expand the idea to saturation, hardness, or even color! Just a thought :)

how about an RMB like menu

-at the center  (instead of colortriangle) -> value
-inner radial  (instead of recent used colors) -> a few fixed quick choices of value ,like 1 ,10,20,50,100,etc to quickselect these rather then dragging.
-outer radial  (instead of brushes )          ->opacity,size,flow,saturation,hardness,etc...



workflow

1) call menu (shift+ctrl +click?) 
2) press the outer radial to choose the active value (opacity,size)
3)drag value by starting at center (may go outside of bounds of the radial menu) to change the value
4)only hide menu when clicking outside of radial menu.


a "select favourite presets"-like manager might be necessary to add ,replace or remove sliders 


maybe similar to https://bugs.kde.org/show_bug.cgi?id=328552
there could be multiple palettes of radial menus with brush values according to categories
 


just a few ideas...

shift+drag could also just be the last chosen value on that radial menu,but then you'd have to show which value is active when sliding. (show 'opacity" or "size")

WISHGROUP: Stretchgoal

Accepted 2015 stretchgoal

This wish report seems like it would use some update regarding what we have in Krita now, how it related to popup palette (especially Bollebib's comment) etc. I do agree it would be nice to have an easier way to adjust the opacity on the fly.