It is possible to create an effect that will automatically outline the content of a layer, and the outline will be dictated by the layer transparency.
How:
- create a layer that will dictate the “shape”
- Put the layer in a Group #1.
- create a Clone layer of the “shape” layer (it’s a special type of layer in Krita)
- Put the layer in a separate Group #2, above Group #1
- Change the blending mode of the clone layer to “Erase”
- Change the Style of the clone layer to “Stroke”
Why:
This special trick takes advantage of the fact that when you use Erase mode inside of a Group, it will not erase content outside of that group, so the clone will be just a void to which you can add outlines. Yay 
Limitations:
A) You can outline just one layer at a time, and the outline will always automatically adjust as you add opaque areas to that layer.
B) Brushes with sharp edges will create a thin outline, brushes with blurry edges will create a smudged outline. Your choice of brushes will heavily affect how the outline works.