PPAPI is a platform for plugins to safely port existing native code, and implement performance critical components. The PPAPI platform capabilities (audio, canvas, webgl, etc) already exist as regular JavaScript APIs in Chrome. So, if you want to use them on normal web content you can. However, in doing so you incur both the strengths and limitations of the web platform (e.g. JavaScript dispatch, IPC overhead, and a single-threaded execution context).

You gave the example of using canvas from a web worker. Unfortunately, the whole web platform is built on the assumption of a single-threaded execution pipeline. Web workers get around this by using postMessage and not having access to the DOM, shared variables, and other stateful parts of the platform. Canvas and WebGl as spec'd and implemented are tied to these stateful parts of the platform (DOM, etc.), which is why you cannot use them from a worker.

So, your desired change would involve either a major overhaul of the web as spec'd and implemented, or a change to the Canvas and WebGL standards. The first is probably not going to happen. The second is achievable and implementable, but wouldn't help address the use cases I described in my first sentence.