Then why have the 'swipe from left' task switching at all?
I agree with jgd12345 - the (dare I say it) iOS style of closing apps would be nice.
For my own app, I've been playing with a custom gesture - place all five fingers on the screen to grab the app, move your fingers closer together as though you are screwing up a ball of paper, then throw the app to the left or right will close it. Sounds horrible in explanation, but it's quite a natural thing to do. Of course, this won't work if someone is missing a finger or thumb, so it would fail in accessability roles!