If you're going to put something like this in game, you need to remember that it is "replacing" the most powerful kill command in the game. Any "stab" idea that involves the victim still surviving the stab is going to need to affect MORE than the victim or it's simply not worth it. A kill secured is worth any stupid gimmick that justifies the use of these (admittedly gorgeous) gratuitous graphics.

You also need to remember that there are already many weapons in place that do a number of things, so you need to make sure that this fills a niche.

A good example: Backstab results in louder-than-normal or longer-than-normal scream, but no body (cowmangler).