How about a sneak-powerattack that you can do with daggers/shortswords? Just give it the basic 2.5x damage multipliers the other powerattacks get, but let this one also have the melee sneak modifier (x6 for example). So in effect it would get a x15 modifier. This would also make daggers/shortswords the weapon to choose for a sneaky melee character instead of a longsword.
This is what they totally should have done for oblivion.
The only issue i have with the oblivion style approach to sneaking was that at 100 sneak you could basically be crouched right in front of someone's nose and still be hidden, which i mean, yeah you'd be good at hiding, but at some point you're gong to fill up your opponent's field of vision and then they have no choice but to see you.
I liked the basic idea of the Mr Sandman perk in FO3, but it was just way too buggy in practice. It would go into the scripted 3rd person animation which was always annoying, you'd suddenly be standing on top of them as they sleep in tehir bed, your head would clip through any obstructions like a bunkbed or whatever, and then it would totally glitch out a times for a bunch of different reasons.
My ideal plan: you get an extra say, 10x or 15x damage multiplier when using a short blade to attack a sleeping opponent when in sneak mode. you get the instant kill effectiveness you'd expect from that sort of attack without a major change to the system they used in Oblivion, plus you don't deal with the bugs that come with some scripted interaction.