Those alternatives are still not what I'm talking about, I'm afraid.
The left side's example of brush opacity is one I took from a very simple online painting program. Painting programs differ in how their opacity works, sometimes. This particular program needs a completely new stroke to go over the previous one in order to get a less opaque color; in some other ones, you just need to go over what you previously colored without lifting your pen/mouse and making a completely new stroke.
What this particular program does with opacity is, after setting the opacity to a lower level (below 100%), there is no need for any further color or brush adjustments: the first stroke is lighter (according to what you set the opacity to) and any subsequent strokes OVER the previous stroke is a shade darker. Eventually if you keep going over it, it will be the color you selected at 100%, in all its intensity. Again, it's very handy for sketching because you can draw out what you want while in a low opacity, so that it's lightly colored, and then you can go over it again bit by bit until you've got what you actually want, and usually when you've gone over it perfectly (aka lineart, or something) the color is the darkest, so you've got a defined drawing.
Without this brush opacity feature, you would need to keep switching colors (ex. sketch with light blue, switch to dark blue and try and go over it again) and doesn't allow much rooms for errors like brush opacity does. Because of brush opacity, you can make multiple sketches and go over the drawing you want over and over, and each time the color becomes a little darker; by the time the color is its darkest (100%), you've probably gotten your sketch perfected.
The following example is done with the same program as above, and then compared to paint.NET:
<---using opacity______________paint.NET--->
I hope you understand what I mean now. Again, to emphasize the usefulness of brush opacity, if you would notice the various hues or variations in intensity in the hand sketch; they are kind of subtle. (Also, pardon if my vocabulary isn't quite accurate considering what is happening to the colors, regarding what hue/intensity/opaque actually is. I'm not very well-versed in these artistic definitions.) The hand sketch was done without touching the opacity button once after setting it to a low level. The lightest color you see is the result of that opacity setting, and my further drawing over that intial sketch caused the colors to get darker because they were lying on top of each other. Eventually I got a finished product that is obviously outlined darker than any previous sketching. If I were to try the same thing with Paint.NET, I would have to keep switching color palettes, and that is much more time consuming and troublesome.
------------ I hope my request is clearer now.
I appreciate the alternative suggestions, but I am simply suggesting/requesting an opacity feature. Not trying to get around it with what Paint.NET already has. Because again, the alpha slide bar is not good for this because it depends on speed for the light/darkness of the color, so if you want a lighter color you have to draw really fast, which isn't helpful at all (and again, the circle marks). The overwrite mode is not what I'm looking for either.