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...
I don't know how easy it is to make such thing, but there certainly are limitations.
I would therefor suggest to make it a 3 pointed curve. 2 points for the beginning and end of the line, and the 3rd in the middle that you can drag to form the curve. This will make the curve to be somewhat static, but it will ensure that it cannot create a curve that makes an impossible envelope.
i already thought about this aswell. i also thought about something similar:
it should be possible to draw such splines in the "visualize effect" window to create nice fadeouts (linear fadeout is not as nice as a x² fadeout)