Just a thought... Making nice envelopes which fade out using a nice curve uses a whole lot of points (and a bit of work) at the moment. It'd be nice if the envelopes could be drawn using splines... Internally, you could still save a "rough" version of the envelope (for example, twenty to fifty points which correspond roughly to the two-point spline you've drawn). I'm thinking of the sort of things you can do in Illustrator/Inkscape... much more freedom!
It'd be even nicer if they were anti-aliased, too...