I know its easy to implement because I did it in a small paint like program. You can repeat a tool's stamp along a line, and I guess it is already the way it is done for pen/brush/eraser strokes between two mouse positions. Because two consequent mouse positions can be distant of a given segment, which can be really long, you have to "fill" between, that is to draw a line of your tool's stamp. With the SHIFT key modifier you only "simulate" a mouse stroke between two distant point. I don't know if I'm clear, but it's easy to do because I guess 98% is already implemented in Paint.NET.
I don't want to draw a line using the line tool. I want to use any tool to draw the line, like the pen, the brush or the eraser tool, or any other future tool. And when I draw, I don't want to switch to line drawing, setting a blending mode, a size or so that is similar to my tool's setting, then drawing my line and switch back to my tool to continue my drawing. This is a question of user friendly interface. I want to use the pen, hold shift and click somewhere to draw a line, and voila.
This is the same user friendly idea that gave us the eraser tool, which is nothing more than a pen with a substractive alpha blending. But an eraser tool is so much user friendly than having to set a negative alpha in the color palette to erase pixels :-)
IMHO, anyone who have done some pixel art or icon drawing or so uses this feature a lot. I do, on Photoshop.
Anyway thank you for this reply, I guess it leads to clarify this feature request.
Bye.