I think is need a reset to default button for Onion Skins tool .
Maybe select to a preset selected.
The reason is to have a range of selected frames by frames .
For example this will make more easy to use Onion Skins tool by each layer into a range of layers.
For example:
layers: sun, sea, birds
If the layer with the sun will need to see few steps ( from : -8 , -4, 0 , 4, 8 )
the layer with flying birds will need more steps ( from -4, -2, 0 , 2, 4)
also all of this changes into Onion Skins tool bars will make a dizzy heights bars , so is need to reset to a preset or a default preset to start another layer animation.