What's truly missing for Wasm and WASI to be an alternative to POSIX is dynamic instatiation so that a Wasm program/component can start another Wasm program/component by providing the bytecode at runtime. So far I don't think anyone is working on that.
if the host provides the guest wasm module via imports a function to create and run from an array of bytes then it can be done today (if I understand you correctly).
Yes, this can be done today, though it needs some gluing together. On the Web (and in Node etc.) you can use JavaScript to create and link the modules, which is how dynamic linking support for wasm works there:
