Hey, though I'm new to the forum, I have been using paint.net for 2 years.
If any one is interested, I have a few recommended plugins
1. On the rounded rectangle, there could an option for the roundness of the corners
2. The other would be a color mixer: you choose 2 colors on the color wheel (or on the picture) and it give you a color "average"
Please let me know if these have already been done, or if they are begun