I found just a few obvious bugs in the test version (paint.net 4.0 alpha build 5086)
but I'm a perfectionist so I added a few suggestions
and divided them on the importance from my perspective
may you find some of them need not only to me.
1) I have not found how to draw a Bezier Curve. (Required)
2) Make the regulator to increase the number of control points of the curve. (Desirable)
3) Clone Stamp Tool does not respond to the value transparency of color, but respond in the last version 3.5.10 . (Required)
Without this can not be done half transparent soft Clone Stamp, it is very necessary for retouching
4) Recolor Tool can not work with transparency, opaque to transparent replace and vice versa. (Desirable)
5) Magic Wand Tool antialiased selection mode not work. (Required)
6) After unchecking the display layer to see the changes automatically allocated the lower layer and continues to
draw on a different layer on which you want. I would like to selection did not move after the layer visibility off, but
maybe I do not know why this is necessary. (Desirable)
7) Make Blend Mode Layer "overwrite" or "alpha mask". Transparent pixels on the upper layer overwrite opaque pixels on bottom layer. (Desirable)
8) Crop to Selection for increase the canvas if you select an area of more canvas (Desirable)
9) After the undo of the action, it remains in the sketch layer. (obvious bugs)
10) As already wrote "quickpost", in Colors window in VHS, "S" and "V" slider changes the original color,
even if his return to the starting position. (obvious bugs)
11) Rotate/Zoom layer, sliders do not affect the ball, can not set a negative value, a little hard to control the ball,
shift does not work with all the axes of the ball, to be honest I would like to back former Rotate / Zoom layer.
(obvious bugs)