I have never seen a "fade-in" or graphical "fade out"
which allows you to draw a line over the area you want
to adjust and then be able to "increase" the volume
or "decrease" it, and yet it is so logical !
Quite often I want to graphically fade a click out of a
section, and then graphically increase the "breath" noise
of a sylible, or the very start of a word that I have
accidentally mumbled. Perhaps doing the both at the
same time, by drawing a line low over the unwanted bit,
and higher over the bit I want to increase. Maybe the
line could work as follows: everything less than 50% of
the height of the wave-window decreases volume, and
everything more than 50% of the height of the window
increases it. (proportionatly, according to just how high
or low you drew the fade/amplify line.
Maybe the volume line could be a permanent part of the
song, being adjusted at will without permanently altering
the original file !
If you really want a high-tech addition, how about
graphically applying effects/eq the same way, selecting
the effect and then applying how much by using a line
superimposed over the area you want it, the higher or
lower the line the more or less of the effect. -Quite often
you want your effects to come in gradually, not with
a "bang", and it is a pain to try to select little pieces at a
time of the song and add progressively more effects to it.
All the best - hope these ideas are useful.