For Animated Avatars we have decided to keep the existing voxel based models. We will split the models into head, torso and limbs, so that these can be separately animated.
We will also be providing a cutdown creative mode where you guys will be able to build your own avatars, it will even support multiplayer so that you can collaborate on a single avatar.
It will allow you to build the model and define what part is the head, torso and limbs, so that the game can apply the correct animation to each part. It will probably also include a preview feature so that you can see what it might look like in game as you build it.
We will be using the very same tool here to animate the built in avatars.
It's possible we might enable this tool for players to create mobs also, but I can't confirm that yet.