Allow users to control interactive animation by layer
Layers could be useful to allow interactions inside the same page, but we also need animation, as we are already using it as an interaction design tool to communicate better some interactions to users. Slide-in/out from borders, scale up/down, modify height/width, move and fade are pretty used today and are needed to be considered as tools from scratch.
Having some control about how layers appear inside of the same screen will give us a lot more options to make good decisions when prototyping.
We already emulate some of these techniques when doing paper prototyping and it's nuts that this animation option isn't being considered in most of the prototyping apps out there.