Check out https://extism.org, it is built for those kinds of use cases. However I think WASI and components could enhance it.

sorry I meant "most excited about", WASI and components should be useful for the usecases I mentioned too.

For example a SaaS services that accepts WASM plugins could provide a WASI that lets the plugin write to a object-store filesystem (like AWS S3) provided by the SaaS owner.