I'm sure it's too late to add to the next version (unless they've done it already), but it's a great idea.
Maybe with the idea that (say) black is 'strong' brush-stroke, and progressively lighter shades of gray contribute less and less. This would let you make a brush which feathered nicely, just by having it 'fade out'. Or a big, strong bold brush which is (in the stencil form) pure black.
Simple, right guys? :-) :-) :-)
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