One of the great things about AVS Video Editor is the large amount of neat transitions, but there aren't too many custom-shape based ones, like Burning Threshold or Burning Fade. Seems like these effects use a Gradient Wipe effect as the base, so how about adding a version that lets the user pick a gradient?
You could limit the gradient image to something already in the media library to simplify the picking process.
Hope you can implement this, shouldn't be too hard (and I would supply LOTS of free gradient templates to make your transition library even bigger).