This is an old thread, but I wanted to add my suggestions to the onion skin request. I do a lot of animating, and prefer AD for character drawing because it is much easier to use than the rather crude drawing tools in many animation packages. Here is what I have to do in order to simulate onion skinning in AD. Just using transparency (alpha) for this is NOT adequate, so if AD had a real onion skin feature, it would be a tremendous help to animators.
1. Make copies of the previous and next cells. This may involve copying many layers for each cell, each with its own gradients, colors, strokes, etc. The next cell is only used when tweening between existing cells, otherwise you only need the previous cell.
2. Reduce opacity for the previous and next cells. Change the stroke colors, often in many different layers. For example, blue stroke for previous cell, red stroke for next cell.
3. Overlay the previous and next cells. Alignment can be very tricky here, if the character is moving. It would be very good if there was a way to define cell boundaries or alignment "pegs" so that the cells could be easily aligned with each other.
4. Create a new copy of the original previous cell (before modifications). Reduce its opacity but keep stroke color the same, as long as it is different from the colors used in step 2. This cell will be the one that is modified to create a tween between the previous and next cells.
5. Align copy from step 4 on top of the aligned previous and next cells. Now you can modify this copy to tween it.
6. When finished tweening, restore opacity (and possibly stroke color) to the new cell. Add it to the animation sequence.
This process works, but is very slow.