Hi @dkazakov Thanks for taking the time. 
About the GUI, I would need more input from others maybe, as I am not sure if we would need much. Maybe a toggle in the menu bar of this brush to activate or deactivate “real paint sampling” vs what we have now “single color sampling”. Only a setting in the brush menu could be also ok for now I guess, since I would make brushes about that functionality mainly, so it would be either on or off for my brush presets.
I personally thing the colorsmudge engine would be the best fit in this case for painting, since that is the tool that looks by far most like “real” paint, especially combined with height texture mode and color smudging. And if one day the smudge alpha functionality is complete, this tool would be even better than the mixer brush in Photoshop. Of course this setting of picked color “fixed” or “scaled” could also be in the clone brush engine that we have now.
I hope I can find a better demo of it, but here you see Craig Mullins using the “mixer brush”. He is pretty much the father of digital painting, he was one of, if not the first person to paint in Photoshop back in the day). If you don’t know him, look at www.goodbrush.com, he is on the forefront of digital painting, always has been.
Yes, it would be great to “keep” the sample exactly like picked, scale as well, as you said it. That would be great. It is not expected nor unexpected, both can be really nice options. Say there is a tiny part in the image where I like the gradient. I can make my brush that size, sample it, make the brush huge and use that as a nice, smooth gradient since it scales the picked image, not only the mask of it.
I am not that good writing these things down, I hope I’ll get more precise and you understand everything somewhat ok.
Also, check this thread from a while ago by @Mythmaker, could help to clear things up on this idea. Experimental Still Life (Clone Brush + Impasto Effects) 🤓