I had an idea about the interpolation that may or may not be useful.. In Illustrator, you can drag directional "handles" on either side of a point to change/add/remove the curve of the line on either side of that point. What if there was a way to do that for the replays as well? ..have a "default" interpolation based on the surrounding path segments, but allow the user to change the strength and possibly direction and speed of the path entering/leaving the keyframe point.