Yung: It may not be what you're looking for, but I was thinking you could have a particle object that moves with the mouse/touch stroke, emitting particles along the way. If you setup the particles correctly, the ones emitted first (at the beginning of the slash) would fade out first. The radius of particles could be small so it doesn't look like a burst but rather just a streak.
Another way might be to have a sprite image that stretches from the initial mouse/touch point to the termination point via scaling or just adjusting the width.