I would be really happy to see some sort of “roundish” shape in addition to the hemite-curve Renoise’s envelopes now utilize as an alternative. Cubic interpolation maybe…?
Hermite curves are a form of cubic interpolation 
Actually, what is used in renoise is Catmull-Rom curves (if I remember the name correctly) which are a subset of hermite curves. One addition I’ve considered is adding a slope parameter for each envelope point, visualized like a tangent line on the curve in each point, which will give control over how fast the envelope curve will change after that point. But as with all other features, it will have to wait for it’s turn.