[..] I was thinking that basically, instead of each time creating a new image every time she can decide to continue on the same image, save the current image and/or restarting from blank.
Currently not very possible unless we define our own image file format (like photoshop or gimp)
Unless you mean that she can just load an image and add her drawings on it, similar to downloading an image from google and making edits (might work)
She could use any library, function or action she wanted, existing or made by us.
see above, this also means the canvas app has to be pluggable which sort of breaks the part where this is supposed to be a simple drawing app
although we probably could drop the "simple" pretense...
I think for this to exist you'd have to either make an app or library that let's you(neuropilot) draw in steps on top of the existing canvas without erasing and then you could create any custom function you want if they don't exist already, or more wackily an entire app with these same features, script parsing, custom commands, etc...
this is literally just coding using turtle but with more steps and in an app not designed to be like that