AE expressions currently support the following interpolation types: linear, ease, ease in and ease out.
To provide better control over the easing curve, I propose implementing an additional interpolation type: Bezier. A matrix of 4 values would be used to define a cubic bezier (similar to how this is handled in CSS animation)
Something like this:
bezier(t, tMin, tMax, value1, value2, [0.4, 0.14, 0.3, 1])
A nice little enhancement, which would allow greater creative control over expression based animation.
Thanks for your consideration.