I think it would be useful to be able to select for each layer whether it will be a target for snapping (default: on); e.g. if layer A has this "snapping" flag set, objects in any layer will snap to objects in layer A. This could possibly be done by a new icon in the layer dialog (along the "lock" and "hide" icons)
Use case:
I want to make a technical drawing where many components will be positioned along an imaginary circle. If I draw this circle, normally I can get objects to snap to it, but as I add more objects to the drawing it will become more complicated to get snapping to work as I want, because different objects will get in the way. By putting the circle in a separate snappable layer and disabling snapping in the layers containing irrelevant objects, snapping to the circle becomes easy again.
A similar effect can be achieved by hiding the irrelevant layers, but this is quite inconvenient and not usable if I want to draw into the hidden layer.
Recently I was looking for a similar feature: I had put labels and figures in a separate, locked background layer to have them visible but not as active snapping targets. But locking a layer apparently doesn't restrict snapping, one has to hide it to prevent snapping to its objects. IMHO implementing a third attribute to layers (hide, lock, snap) would be a useful enhancement.