Okay - it’s taken a while to write this so sorry if it seems out of step with the flow of conversation, or I’m repeating what’s already been said!.. 
Re: What brush engines to enable with; The current clone brush behaves like a pixel brush anyway - just with very limited options. The results can look very painterly when using the right brush tips and settings.
As I used it with the layered impasto effect (in real-time) in those two images I did, the feedback was more satisfying as I was painting; so I’m pretty hopeful that combining that ability to sample with the colour smudge + lightness (if possible) should produce some really nice results.
Even with only the current clone options available, it would be useable. But I’ll try to lay out what features I think would be useful.
I agree about being able to hide the sample point - mainly because if you sample close to where you’re painting it can obscure the area you need to see.
Useful options for the sample point:
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Toggle for fixed sample size/scale with brush.
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Toggle for fixed rotation/rotate with brush.
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The ability to combine the sample with the current selected colour, and a slider to change the ratio of the mix. I think that’s similar to how Photoshop’s mixer brush works.
I’m trying to remember what the limitations currently are for where you can sample from. I know you can sample from other layers, but I think it’s hard because of the key-combos required. So maybe some kind of lock/toggle for the source layer.
On that subject, it would be good to be able to sample from another image (can’t remember if that’s already possible) - so you can keep whatever you’re sampling from out of the way e.g. a painted-out palette, or a photograph. (That’s if @halla doesn’t mind people abusing the feature to turn photos into paintings!.. 
).
By the way - We had some discussion around this about a year ago in this thread. There might be some useful stuff in there; I did do some research and experiments back then.