As a professional animator I have 4 requests (in order of my suggested importance) that would make this the ultimate gaming engine. I truly believe that adding these features would blow the competition out of the water.
1) Deformation - I saw in your videos that your guys are considering adding a deformation type tool. It doesn't need to be that complicated during the beginning of your project. To get the ball rolling I suggest adding the ability to manipulate only the 4 corners of every limb image. Having the ability to manipulate the images to the extent shown in your videos is great, but I think you will be very happy with what animators can do with just manipulating the four corners. (might be easier to implement aswell)
2) Easing - Easing in your tweens helps tremendously when attempting to make your animations look more believable. A simple -100 to 100 option(similar to flash) is sufficient. -100 being ease in and +100 being ease out. Maybe an option to have both ease in and out in a single tween would be helpful too.(just a thought)
3) Dynamically adding sprites - This one is obviously harder to implement, but maybe consider it for a future tool. The simplest way i can think of it is a single color that you can draw and erase to dynamically add limbs/sprites/whatever. It doesn't need to have all the whistles of a painting program, just something that allows you to quickly add elements that you can later take to a separate painting program to make pretty. (Great for prototyping animations).
4) Offsetting Limbs - (This might already be possible, not sure) When animating characters without a skeletal structure (a lot of flash animation is done this way) it would be nice to offset the animation of separate limbs via shifting the keys over a few frames. This can help speed animation workflow. (Common in many animation programs)
Thanks for hearing me out!