So many features of SL are obviously modelled on Photoshop-like methods. One useful tool set that might be added some time would be the concept of layer masks for selection areas and where the mask luminosity/opacity is the feathering value. Right now feathering/fade applies to edge selections, but with a gradient tool that could be made to apply to a selection area in new ways.
For example, making a rectangular selection on two layers with the gradients in opposite directions for each layer can make a nice spectral morph - the fade on one layer decreases and the fade on the other channel increases.
There are many useful techniques that can improve on the fade/feathering we now have.