I think that for some interactive content, we could do definitely utilize WeP's lossy alpha channel format. The one thing that pops up in my mind are filmstrip animations, where you make a animation using JS and a single image that has all your frames. This allows you to control its playback with JS and gives you better image quality than GIF. Right now we can use JPEG compression for this, but this means we have to bake the page background into the frames or use a huge PNG.

With enough browser support an animation like this could just fallback to a single PNG for older browsers and newer browsers would be better experiences.