A varient which might cover the points both of you have raised is to have a very simple shield that then provides a connector (outside of the stacking profile) for a power module that has the MOSFETs, etc. That way you can have one shield that can you can plug different rated (10A, 30A, 100A) power modules into and down the track probably simpler AVR (or other microcontroller) boards that the power modules can plug into when you don't need the stacks. The power modules could be 'side-cars' to the shield
The thing I don't like about this is that the shield is very basic, I don't think it would really be much beyond a connectors/tracks and I think in most cases you are probably going to have a single ESC made out of one power module and one micro. In that case it just adds cost and probably space for little gain. For example there are not enough ADCs and timers in general to do more then one ESC per AVR.
If you look at commercial ESC they are pretty small, for example I have a 30A one with all the power fets on one side which nearly fix in the square profile of the TinyDuino and it uses 5 fets per phase rather then just two which clearly fit inside the profile. The other side has the AVR, etc.
I think we could for 10A probably do a single shield (no stacking the MOSFET packages are too high and probably need a heatsink), in the standard footprint. I might look at how feasible such a design is. I think both a straight (top of the stack) shield and power module side-car shield have their usage in different domains or stages of development at any rate.