Are you planing to add vector shapes support in the future? It can't be done in pure CSS I guess, but it can be SVG based, if I'm right. The possibility to control each separate control points of paths, and individually animate them would be very useful. It would be also nice to have some way of visual identification of each control point (numbers, colors, both). Thats because their location may change in time and it might be confusing to determine 'who is who'. This would also help Sencha Animator to determine how they animate. In Flash i.e. you got system of "shape hints". while its very useful option, it's bit impractical and slow in use. Theres got to be a way to make it more user friendly.
The other option that I want to ask about is something like "basis layer". This kind of layer would only store movies or flash swf's, and wouldn't be part of animation/project. It would be there only for being basis for recreating flash/movie based animation in your program. This way, you could animate in Sencha Animator on top of running swf or movie file. It would greatly improve speed and accuracy in process of converting flash content to open technologies.