Keyframe can contain expressions and that keyframe's value is the result of the expression
Allow keyframes to contain expressions. This would allow the user to blend hand-crafted animations with procedural parts.
Brian Carter commented
I wasn't particularly verbose in my original request so I'll explain more here.
Sometimes, in animations, there are certain values that are not creative as much as they are derived. "At frame 10, this object should land where this other object is." "The first keyframe of this layer should be positioned offscreen the width of the layer, so it always enters from off the frame, even if we change the size of the layer."
It is possible to use expressions to make these decisions, but if a property is controlled by expressions its controlled entirely by those expressions and no manual key framing is possible without creating hybrid systems using expression controllers and conditional expressions.
I am proposing that keyframes can be double clicked to edit and in the resulting dialog there is an expression window that allows the user to enter an expression that can be evaluated only for frames involving that keyframes value.