Dude the crit wound idea is so brilliant. A 10 second lv3 Crit Wound could be super crucial in defeating a group without one shotting them - or with the combination of the range thing, closer people get Crit Wound lv5 while further people get up to Crit Wound lv2. Lol, crazy coding though, not likely to happen.
What about, after you get SD'd, you get a buff for a few seconds where another SD does only 50% on you, and decreases further more every time, so it decreases efficiency on multiple SD's? (But then it doesn't make sense, especially when the first SD usually kills you)
And as Kevin mentioned a change outside of the skill itself is a good way of thinking too, since that is more likely to happen