Bug 395928

Adding this feature would let Krita become a more professionally viable animation software. It allows for animators to move the onion skins so that they can in-between their drawings in a custom position, allowing inbetweening to become significantly easier. This is a common practice, especially in the Japanese and Korean industries. 

There are two ways to go about this. Most of your current audience would prefer #2, but older, traditional animators are more familiar with #1:

1) (seen in Clip Studio Paint, Toon Boom Harmony) Ability to select a drawing, or the current image, and send it to a "table". This creates a custom onion skin view where you can transform those selected onion skins.

2) (seen in TVPaint) Ability to affect onion skin images's transformation within the existing onion skin view. 

https://www.tvpaint.com/doc/tvp11/index.php?id=lesson-tradigital-animation-advanced-classic-tools-oop

https://docs.toonboom.com/help/animate/Content/HAR/Stage/005_Traditional_Animation/272_H2_Shift_and_Trace_.html