This is a funny card I thought of some time back: Miss Lethal.
Essentially, it's just a way to find out if you missed lethal at any point of the match.
Mechanically, I can see it working like this: At the point where you end the turn, if there was some card on the board or in your hand that could be played to kill your opponent that round, then she would appear, otherwise she would not. This does not take into effect RNG-based effects, of course.
If you never miss lethal during the match, then you would run a deck with just 29 cards, which is useful by itself for OTK decks that want to make themselves more consistent with their drawing.