An internal rack modual that one could create their own 'groups' and select a key or multiple keys to routed to their own effects chain with-in the rack.
So in it's basic form you could have say a rack with an Impulse, then the 'Key routing modual'. Create a group called say 'snares' (within the Key routing modual), right click on that group and up would show the keys to choose from (the white keys from C3-C4 in this case) then you could choose one or multiple keys to route to this group. Each group would then act like a chain that one could add effects to.
There is also these threads
http://www.ableton.com/forum/viewtopic.php?t=47624
http://www.ableton.com/forum/viewtopic.php?t=48182
Though I feel these are more about choosing a whole chain to route to a track (though there is some stuff about routing from groups of Impulse pads/keys.
I want to be able to select multible keys - be it from a multi sample Sampler instrument, Impulse pads or other vst/au.
I feel this would also solve some issues with Sampler not have individual effects/controls for each key/sample.