First: StrokeIt is great ... :)
So here's my suggestion: How about an option to restrict gestures to parts of the screen?
Eg how about dividing the screen into four segments and making this an additional parameter for actions, so an "S" in the upper left region could be used for "Save", while an "S" in the lower right region would trigger Slashdot's URL.
This woud quadruple the number of gestures without the necessity for modifier keys. And if it were an option, people who don't want it wouldn't have to bother.