You can already do this manually right now using one of the Color Smudge Engine brushes. I personally really like the Wet Bristle brushes for any sort of painting requiring blending.
I recently revived a long standing request for a scratchpad docker that would allow us to dedicate some space on a side bar to use for more traditional style color mixing. You can find it here: Scratchpad Docker
The somewhat clunky approach right now is to use some unworked corner of your canvas or a dedicated layer to drop down some color dabs and do your mixing. But then you are stuck having to toggle that layer’s visibility or zooming/scrolling to reveal the area where you were doing your mixing any time you want to sample colors again. Another alternative, if you generally don’t need access to the UI much is to open up a second file and go into canvas only full screen mode with the tab key. If you are using sub-windows instead of tabs you can arrange your main canvas and your mixing canvas side by side, with the mixing canvas placed off to the side where the side bar usually is. You will have to tab back to the UI any time you want to mess with layers or switch to other tools that you may not know the shortcuts for, but it’s better than nothing if you really like to mix your own colors by hand, at least until if/when that docker becomes a thing.