Here’s the answer:
Onion skinning is by far the most requested feature for After Effects. It’s also very high on our list of priorities to work on for a release that we’re targeting
for the second half of next year.
I’ll be posting the list of the top 50 most requested features on the After Effects team blog this weekend, complete with commentary about each:
http://blogs.adobe.com/aftereffects
Interesting. Better paint features (ie vector paint)? Node compositing features? A "make awesome" button?