I highly agree with you. DrawingNow's application really needs an eraser, blend/smudge tool, etc. Especially an eraser, so there wouldn't be anymore mistakes on drawings.
Now about the paper backgrounds and the opacity, I thought there's already one. When I tried looking up on the background while on DrawingNow's applications, the background really does look like a paper.
And the layers, I definitely agree with you. Because when I tried drawing and drawing on the application, the color turned out runny like watercolor. But there may be already layers for the drawing of Drawing and Sketch/Guide lines, but my opinion, they're not the best layers.
And by the way, I also added an extra idea that would be good and bad. The ink should be removed so there would be no chance to stop drawing on there anymore. But I wouldn't think it's a great idea, because the more ink on drawing, the more longer the drawing animation would be when posted. And then it would lag the internet/computer.