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I already consider kolourpaint to be superior to any version of mspaint, but I see 3 minor aspects that kolourpaint lacks:

1) A full-screen mode that takes over the entire screen. I think mspaint calls this "View Bitmap". Clicking the mouse would take it out of this mode. The ability to make a screen display a picture quickly and without any hassle would be useful.
2) Ctrl scrolling (either with +/- or the mouse-wheel) in mspaint would make the brush sizes change, allowing ridiculously large brushes, and changing brush sizes while drawing*. I also like that Ctrl scrolling zooms in kolourpaint, so having both functionalities somehow would be awesome.
3) Kolourpaint isn't included by default on any of the live DVDs I've tried. Kolourpaint is my favourite KDE program (that I'm aware of), and things like GIMP are too different.

1 & 2 might already be there somewhere, and there might be a live image with kolourpaint on it, but I haven't found them.

*https://www.youtube.com/watch?v=TBK7XRb2JDs&t=2m18s - I was sufficiently disappointed with Windows 7 paint that I got Vista's version to replace it.