Good idea, but not with Synfig, because when you cut a spline then in Synfig it would be represented by two different Outline layers, and merging them would make them one layer. So maybe we should make the spline more like Standard SVG and then implement this feature.
Like implementing this
We should focus on curve editor, which would actually have spline curve interpolation. Adding this would make the animation much smoother and give more control to animator.
Also the same idea for posting/exposing Synfig actions to Python (or Javascript). And provide an interactive way for plugins to interact with the user.