Not sure if this has been requested already but a mid-side unit would be great.
One implementation in terms of the UI is to have two parameters with a level control (just like a mixer) for mid & side. Then you can go into each parameter & add whatever processing you desire on the mid & side independently. The key thing is that the forward stereo-to-M/S and the reverse M/S-to-Stereo transforms are implicit in the unit I.e. the input & output of the unit is stereo & the forward & back M/S decoding is hidden from the user.