I know that Paper2D currently has the concept of a flipbook which lets you create an animation from key frames, but I think this is too limited in some cases.
What I’m instead is proposing an animation system where each frame is created by placing various sprites into the frame at a relative location. You would also be able to set the scale, rotation, and color of each placed sprite in the frame.
This would allow a much more diverse range of animations to be created from existing sprites rather than creating new ones. It would also be able to achieve the same type of animation as the existing flipbook system.
Additionally, I think it would be desirable to be able to override the sprite used for a particular animation instance at runtime. Why? As a matter of customization. Think about a 2D RPG. You could replace various parts of a character like the head or body depending on what you’re wearing with this override functionality.