In Paint.NET, one of the features that I use most consistently that’s also missing from even the most advanced editors is the ability to preview certain tools before confirming them.
This allows one to make modification to the tool before actually applying changes. This is very useful, for example, to visualize the sweet spot in tolerance-driven tools like the paint bucket tool and the magic wand (Additionally, Paint.NET also has tolerance as a slider, which makes seeking the right tolerance trivially easy in complex images, instead of having to manually input numbers which makes it easy to miss less common values)
This is a fairly fundamental aspect of my workflow and I would love to see it implemented.