I would like to suggest a new feature, where you can choose whether the onion skin effect is rendered 1) on top of the camera view (ie on top of all layers, regardless of where in the stack or z-axis they are), 2) just above the active layer, or 3) just bellow the active layer like current behaviour. This would negate the need for a lot of layer hiding/showing and messing around with which layers are displayed as outlines, and let you animate in context, but with full view of the onion skin guides.