Sorry dude, I didn't read your whole list (which was an AWESOME list btw), so you might have already written this ability down.
But here goes:
(I just thought of this in the shower )
Assassin: Drasticallyincreases the accuracy of one-hit KO moves. So basically, this changes the accuracy of moves like Guillotine from 30% to 80%.
(80% sounds a little too accurate, maybe it should be changed to 70%)
I even came up with a pokemon with this ability:
I thought it was a cool name, it's design sorta looks like a cross between the creature 'venom' from spiderman, and sableye. This creature has a long, furry tongue that hangs out from it's Sableye - like grin. It has blank, soulless blue eyes (like Sylveon), and has long, black scythes that replace it's hands (like Kabutops's blades - except longer and black).
Now it's stats:
Base stat total: 440
You are probably disappointed with it's attack stats, but when it has the ability 'Assassin', who needs attack? Zlythax can learn ALL the OHKO moves, including a new one called 'Death Slice', a dark type guillotine which is it's signature move.
Since Zlythax has a stunning 150 speed stat, it should OHKO everything in it's path, at least 80% of the time!
(Heh, all this text to explain one ability I came up with )
Sorry, that was a little off-topic. But I wanted to explain to you my creature in detail, and it's ability.