sketching on top of an existing image as a reference would be great :)
You already have a background layer that can be changed without erasing the sketch, just need to be able to load my image from photo album as a reference. Then I can replace it before saving with a different background or paper. It would also allow for sketching over a previous sketch to avoid getting unwanted connecting lines for fine details
This would be extremely useful when you find you want to say, draw an outline over the top of what you already have, so it won't just make a huge splotch. Another option to disable the automatic shading (this is the very nice feature, but there are situations where you would like a clear line either over the top of existing lines or close next to them) could also resolve this issue.
The great about the app is that the sketch interacts with pre-draw lines, however sometimes it is difficult to have sharp lines because the context is too much busy, thus having layers that does not interfere with each other will be a great plus
Moreover it will be also great to have the ability to define a different scale for the effect for each layer.