The problem is a lot of the features <canvas> gives aren't easily implemented into webgl, things like paths, curves etc.
On the other hand some of the functionality you are looking for already exists, but its pretty limited, IE snapshot canvas.
The question is what other functionality is available?
Or is there some happy medium, such as the ability to snapshot certain areas of the screen, and or if we aren't able to draw curves, could we perhaps draw lines, or even just plot pixels like in Construct Classics Canvas?