Precisely! and that challenge would be achieved through smart stealthy gameplay!!
Being devious, clever and sneaky in the challenge rewards the player. The learning curve comes in when the player explores the AI routines, the environment and different ways to hide the bodies....the player learns how to effectively kill a bodyguard without leaving a trace.
This learning curve is an intrinsic, natural one....not imposed by mechanic restriction
Similarly, in case of M&E, the learning curve comes into play when the player is encouraged to use it in situations which require planning and observation.
GRFS is a good example of how the M&E learning curve should progress. In the initial missions the Sync Shots are easy to pull off..but as you progress you get more enemies, complex patrols, varied geometry etc. It's a great way to show that M&E can be an extremely tactical tool with a great learning curve IF the mission design and AI cater for it.
Another thing which I have against pre-packaged M&E diificulty is the fact that it eliminates certain hybrid playstyles and stomps out gameplay experimentation.
I'd like to try an aggro-ghosting experience, combining M&E into the classic stealth approach. I would use it to move around and silently eliminate key enemy personnel all the while making sure that nothing is traceable. A quick silent attack followed by hiding the evidence.
If we were to go the standard difficulty route you suggest, it might not be possible to do so...given that a Ghosting experience would require the AI to be super sharp hence the hardest difficulty...where in the M&E would be severely limited. I would not enjoy going through this hard as nails loop.